THE DECENNIAL PUBLICATIONS OF 
THE UNIVERSITY )F CHICAGO 



THE STUDY OF STELLAR EVOLUTION 



HALE 















•\ 



^ c- . -/' 







'^^. .^"^ 



'^^> ■< 










THESE VOLUMES ARE DEDICATED 

TO THE MEN AND WOMEN 

OF OUR TIME AND COUNTRY WHO BY WISE AND GENEROUS GIVING 

HAVE ENCOURAGED THE SEARCH AFTER TRUTH 

IN ALL DEPARTMENTS OF KNOWLEDGE 



THE STUDY OF STELLAR EVOLUTION 



PLATE I 




The Great Nebula in Andromeda 
Photographed with the 24:-inch reflecting telescope of the Yerkes Observatory (Ritchey) 



THE STUDY OF STELLAR 
EVOLUTION 



AN ACCOUNT OF SOME RECENT METHODS 
OF ASTROPHYSICAL RESEARCH 



GEORGE ELLERY HALE 

FORMERLY OF THE DEPARTMENT OF ASTRONOMY AND ASTROPHYSICS 
NOW DIRECTOR OF THE MOUNT AVILSON SOLAR OBSERVATORY 



THE DECENNIAL PUBLICATIONS 
SECOND SERIES VOLUME X 



CHICAGO 

THE UNIVERSITY OF CHICAGO PRESS 

1908 



0? xx 






LIBRARY Of CONGKESS 
[wo Copies Received 

MAY 27 1908 

wu»>j)<iAru entry 

OUSSk /V XXc. No. 

COPY B. 



Copyright 1908 by 
The University of Chicago 

Entered at Stationers'' Hall 



Published May 1908 



Composed and Printed By 

The University of Chicago Press 

Chicago, Illinois, U. S. A. 



PREFACE 

As first planned, this book was intended to serve as a 
handbook to the Yerkes Observatory. Many inquiries 
regarding the observatory's work, made by the numerous 
visitors received there annually, seemed to call for a printed 
explanation of the purposes in view and the observational 
methods employed. Removal to California and new duties 
connected with the organization of the Mount Wilson Solar 
Observatory caused a modification of the project. I finally 
adopted the plan of describing a connected series of inves- 
tigations, laying special stress on the observational methods 
employed, in the hope of explaining clearly how the problem 
of stellar evolution is studied. The advantage of using con- 
crete illustrations drawn, in large part, from personal experi- 
ence, and the desire that the book should be of special service 
to visitors at the Yerkes and Mount Wilson Observatories, 
are sufficient reasons, I trust, for the otherwise undue pro- 
portion of space devoted to these institutions. 

The omission of such important subjects as the theories 
of temporary and variable stars; Sir George Darwin's dis- 
cussions of evolution as affected by tidal friction; Vogel's 
and Pickering's photometric and spectroscopic studies, and 
the researches of the latter on the distribution of stars of 
various types; Campbell's investigations of stellar spectra, 
and, to mention no other work, his development of the spectro- 
graphic method of determining radial velocities, sufficiently 
indicate that I have made no attempt to deal with the general 
problem of stellar evolution, or to offer anything approaching 
an adequate description of the observational methods of astro- 
physics. The various researches described are chosen rather 
arbitrarily, in some cases with more regard for my personal 



Preface 



acquaintance with the facts than because of their intrinsic 
importance. I trust, however, that although this method of 
treatment has necessarily resulted in a fragmentary exposi- 
tion of the subject, the book will serve to show how the 
problem of stellar evolution is attacked along converging 
lines, leading from solar, stellar, and laboratory investiga- 
tions. 

I wish to express my thanks to Sir William Huggins ; to 
Messrs. Adams, Ellerman, Olmsted, and Ritchey of the 
Mount Wilson Solar Observatory; to Professors Barnard, 
Burnham, and Frost of the Yerkes Observatory; to Professor 
Campbell of the Lick Observatory; to Professor Pickering 
of Harvard College Observatory ; to Mr. Abbot of the Smith- 
sonian Astrophysical Observatory; to Professor Lord of the 
Emerson McMillin Observatory ; and to Professor Ames, 
Mr. Jewell of Johns Hopkins University, for photographs 
which appear in the plates. I am also indebted to the 
Astrophysical Journal and the Publications of the Yerkes 
Observatory for many cuts, and to Messrs. Ticknor & Co. 
for permission to reproduce Langley's drawing of a typical 
sun-spot. I am under special obligations to my colleague, 
Mr. Ellerman, to whom are due most of the photographs 
of instruments, buildings, and landscapes which appear in 
the plates, besides many of the solar and stellar photographs 
taken from our joint papers in the Astrophysical Journal 
and elsewhere. 

G. E. H. 

Pasadena, California 
November, 1907 



CONTENTS 

CHAPTER PAGE 

I. The Problem of Stellar Evolution - - - 1 

II. The Student of the New Astronomy - - - 9 

III. The Sun as a Typical Star - - - - - 15 

IV. Large and Small Telescopes - - - - 20 
V. Astronomical Photography with Camera Lenses - 27 

VI. Development of the Reflecting Telescope - 38 
VII. Elementary Principles of Spectrum Analysis - 46 
VIII. Grating Spectroscopes and the Chemical Compo- 
sition OF THE Sun ------- 56 

IX. Phenomena of the Sun's Surface - - - - 67 

X. The Sun's Surroundings ----- 73 

XL The Spectroheliograph ------ 82 

XII. The Yerkes Observatory ----- 97 

XIII. Astronomical Advantages of High Altitudes - 111 

XIV. The Mount Wilson Solar Observatory - - 121 
XV. The Snow Telescope ------ 131 

XVI. Some Uses of Spectroheliograph Plates - - 139 

XVII. A Study of Sun-Spots ------ 151 

XVIII. Stellar Temperatures -----. 165 

XIX. The Nebular Hypothesis - - _ _ . 175 

XX. Stellar Development - - - - - - 187 

XXI. The Meteoritic and Planetesimal Hypotheses - 204 

XXII. Does the Solar Heat Vary? - - - - 212 

XXIII. The Construction of a Large Reflecting Tele- 
scope --------- 219 

XXIV. Some Possibilities of New Instruments - - 230 
XXV. Opportunities for Amateur Observers - - - 243 



Index 



251 



CHAPTER I 
THE PROBLEM OF STELLAR EVOLUTION 

It is not too much to say that the attitude of scientific 
investigators toward research has undergone a radical change 
since the publication of the Origin of Species. This is true 
not only of biological research, but to some degree in the 
domain of the physical sciences. Investigators who were 
formerly content to study isolated phenomena, with little 
regard to their larger relationships,. have been led to take 
a wider view. As a consequence, the attractive qualities of 
scientific research have been greatly multiplied. Many a 
student, who could see in a museum only a wilderness of dry 
bones, now finds each fragment of profound interest if the 
part it plays in a general scheme of evolution can be made 
clear. The color and structure of any animal or plant, the 
minute modifications which distinguish one variety from 
another, take on new significance when considered as evi- 
dences of development. Their appeal to the microscopist, 
or to anyone who finds delight in intricacy of structure or 
beauty of form, is quite as great as before. But to the stu- 
dent whose interest is not aroused by such details, perhaps 
from lack of technical knowledge, or from the feeling that 
these matters are trivial as compared with the larger problems 
of science, such minor peculiarities must appear in a new 
light. Their true significance becomes apparent, and the 
importance of studying them, once perhaps underestimated, 
now requires no demonstration. 

In astronomy the idea of evolution goes back to a very 
early period. In a crude and grotesque form the traditions 
of the earliest peoples invariably struggle to account for the 

1 



Stellae Evolution 



origin of the Earth and its inhabitants. On a much higher 
plane stand the speculations of the Greek philosophers and 
of those who have followed them in the centuries preceding 
our own time. All schools of astronomers, dealing in some 
instances with purely philosophical and theoretical considera- 
tions, and in others basing their conclusions upon known 
facts of observation, have sought in their turn to explain the 
origin of the solar system and the larger relationships that 
obtain in the universe as a whole. In the eighteenth century 
these speculations reached their climax in the nebular hy- 
pothesis of Laplace, which still remains as the most serious 
attempt to exhibit the development of the solar system. 
Attacked on many grounds, and showing signs of weakness 
that seem to demand radical modification of Laplace's original 
ideas, it nevertheless presents a picture of the solar system 
^which has served to connect in a general way a mass of indi- 
vidual phenomena, and to give significance to apparently 
isolated facts that offer little of interest without the illumina- 
tion of this governing principle. 

It will be seen, therefore, that the idea of evolution and 
development is by no means new to the astronomer. But it 
may nevertheless be maintained that it has occupied a more 
important position since Darwin published his great work. 
^In 1859, the very year of the publication of the Origin of 
Species, Kirchhoff first succeeded in determining the chemical 
composition of the Sun by the aid of the spectroscope. His 
fundamental discovery marked the entrance of this instru- 
ment into the field of astronomical research and established 
on a firm basis the new science of astrophysics. The impor- 
tance of spectroscopic investigations in their relationship to 
evolution was soon made clear. Within a single decade the 
study of stellar spectra by Huggins, Rutherfurd, and Secchi 
had shown that the stars may be divided into several classes, 
characterized by distinctive peculiarities in their luminous 



The Problem of Stellar Evolution 3 

emission and marking definite stages in an orderly process 
of development. Following close upon this pioneer work 
came the capital discovery by Huggins of the gaseous nature 
of the nebulae, and the relationship of these celestial clouds 
to the stars which they enshroud. In these filmy masses of 
luminous gas it appeared probable that the stars had their 
origin, taking form after long ages of condensation, through 
processes regarding which our ideas are still vague and ill 
defined. Belief in such a mode of development has been 
greatly strengthened through the results of recent investiga- 
tions, and especially through the discovery by Keeler that 
of 120,000 nebulae strewn over the heavens fully one-half are 
distinctly spiral in form. This far-reaching conclusion, 
coming at the end of the nineteenth century, is furnishing 
materials for those who seek, through modification of the 
nebular hypothesis, to provide a sound and sufficient explana- 
tion of the development of suns like our own. 

We are now in a position to regard the study of evolution 
as that of a single great problem, beginning with the origin 
of the stars in the nebulae and culminating in those difficult 
and complex sciences that endeavor to account, not merely 
for the phenomena of life, but for the laws which control a 
society composed of human beings. Any such consideration 
of all natural phenomena as elements in a single problem 
must begin with a study of the Sun, the only star lying near 
enough the Earth to permit of detailed investigation. The 
knowledge thus derived may then be applied in researches 
on the nebulae, and in the elucidation of spectroscopic obser- 
vations of those stars which represent the early period of 
stellar existence. According to present views, the state of 
development attained by the Sun is that of maturity, if not 
of decline. After it come the red stars, which represent the 
last stages of luminous stellar life. Even the extinction of 
light due to continued cooling is not sufficient to exclude 



Stellae Evolution 



altogether from the astrophysicist's study those dying stars 
which represent a condition lying between that of a glowing 
sun and a dead planet like the Earth or the Moon. Through 
one of its many remarkable properties, the spectroscope 
enables us to detect the presence, and sometimes to deter- 
mine the dimensions, of vast bodies which have resulted from 
the cooling of former suns. It will be the object of this 
book to show how the student of astrophysics attacks this 
problem of stellar evolution, through the development of 
special instruments and methods of research, and the accu- 
mulation and discussion of observations. 

It must not be forgotten that such a study comprises only 
the earliest and simplest elements in the general problem of 
evolution. The province of the student of astrophysics may 
be said to end with an understanding of the production of a 
planet like the Earth. It remains for the geologist to explain 
the changes which the surface of the Earth has undergone 
since the constructive process left it a rocky crust. The con- 
ditions which brought about the formation of the oceans, the 
effects of the long-continued action of winds and waves, and 
the vast changes in surface structure that have resulted from 
internal disturbances and the operation of volcanic phenom- 
ena, afford limitless opportunity to the student of evolution 
in this other aspect. Closely related to these changes, and 
presenting difficulties far greater than those experienced by 
the astrophysicist, comes the problem of accounting for the 
origin and development of plant and animal life. The pres- 
ervation of the earlier forms of life, principally through the 
agency of sedimentary deposits, affords the paleontologist the 
means of connecting the links in the evolutionary chain. 
Thus we are brought to our own era, where countless living 
objects continue to supply material for new inquiries. Both 
in the examination of existing species and their relationships, 
and in those experimental researches on variation which offer 



The Peoblem of Stellar Evolution 5 

such promising opportunities to the investigator, the evohi- 
tionist may secure data for further advances. Outside the 
immediate domain of the natural sciences, in regions of 
activity where still greater complexity prevails, the student 
may seek to trace out evidences of unity and development in 
the mental and moral relationships of the peoples of many 
countries and of many generations. 

It is a noteworthy fact, of prime significance to all investi- 
gators who find special interest in attempting to enter new 
and unoccupied fields, that some of the most important devel- 
opments of recent years have taken place in those regions 
which lie between the boundaries of the old established 
sciences. Thus the union of physics and chemistry has 
opened up the extensive field of physical chemistry, where 
advances of the greatest value are being made. In the same 
way the application of physical methods and the principles 
of physical chemistry to the experimental study of physiology 
has resulted so successfully as to give hope for even more 
remarkable developments in the near future. In astronomy, 
the introduction of physical methods has revolutionized the 
observatory, transforming it from a simple observing station 
into a laboratory, where the most diverse means are employed 
in the solution of cosmical problems. The fact that physics 
is common to these and other intermediate branches of 
science affords striking proof of its fundamental importance. 
An investigator who has been confined to the traditional 
methods of a department of science where physics has as yet 
played little part, may therefore find in physical methods a 
powerful means of advancing his subject. 

The suggestive value, to investigators in other depart- 
ments, of any species of scientific research which involves new 
methods and principles is perhaps greater at the present 
time than ever before. Even those methods of research 
which can find no direct application in other subjects are 



6 ■ Stellar Evolution 

frequently capable of suggesting modifications or adaptations 
involving related principles. The development and use of 
new methods is quite as likely to advance a subject as the 
prosecution of extensive investigations by existing means. 
For this reason the investigator is ever on the alert to seize 
and utilize suggestions derived from any source. 

The interest of the student of astrophysics is no longer 
confined simply to celestial phenomena. For astrophysics 
has become, in its most modern aspect, almost an experi- 
mental science, in which some of the fundamental problems 
of physics and chemistry may find their solution. The stars 
may be regarded as enormous crucibles, in some of which 
terrestrial elements are subjected to temperatures and pres- 
sures far transcending those obtainable by artificial means. 
In the Sun, which appears to us not merely as a point of 
light like the stars, but as a vast globe whose every detail 
can be studied in its relationship to the general problem of 
the solar constitution, the immense scale of the phenomena 
always open to observation, the rapidity of the changes, and 
the enormous masses of material involved, provide the means 
for researches which could never be undertaken in terrestrial 
laboratories. Hence it is that astrophysics may equally well 
be regarded as a branch of physics or as a branch of astron- 
omy. A telescope may be defined as an instrument for 
revealing celestial phenomena, or it may be likened to the 
lens which the physicist uses in his laboratory to concentrate 
the light of an electric spark on the slit of his spectroscope. 
To the student of astrophysics whose interests are not con- 
fined to a single branch of science, the subject is likely to 
make a double appeal, no less strong on the physical and 
chemical than on the astronomical side. 

In entering upon our consideration of the study of stellar 
development, we may think of the subject in either one of 
two ways. Some will prefer to regard it as the general prob- 



The Problem of Stellar Evolution 7 

lem of stellar evolution, in its broad application to the uni- 
verse at large. But others will find it easier to conceive of 
the question as an investigation of the Sun, tracing it, 
through analogies afforded by stars in earlier stages of 
growth, from its origin in a nebula to those final chapters 
which, though not yet written for the Sun itself, may be read 
in the life-histories of the red stars. Viewed from which- 
ever standpoint, the task of the investigator remains the 
same, since in either case it is concerned with stellar origin, 
development, and decay. 

It must, of course, be remembered that the processes of 
stellar development ordinarily advance so slowly that a life- 
time would be far too short to permit any permanent change 
to be observed in a star. Temporary stars flash into view and 
fade rapidly away; but these represent an abnormal condi- 
tion, typical of some catastrophe rather than of a natural 
course of change. The spiral nebulae, though their appear- 
ance leaves little doubt of extremely rapid motion and con- 
stant change of form, are so far removed, and constructed on 
so vast a scale, that no actual differences in structure have 
been detected in photographs of the same object, taken at 
intervals of many years. In the processes of creation a thou- 
sand years is but a day, and we must be content to base our 
stellar histories upon analogy. 

Fortunately, the data needed for the construction of these 
histories are easily found. Our problem is like that of one 
who enters a forest of oaks, and desires to learn through 
what stages the trees have passed in reaching their present 
condition. He cannot wait long enough to see any single 
tree go through its long cycle of change. But on the ground 
he may find acorns, some unbroken and some sprouting. 
Others have given rise to rapidly growing shoots, and sap- 
lings are at hand to show the next stage of growth. From 
saplings to trees is an easy step. Then may be found, in the 



Stellar Evolution 



form of dead limbs and branches, the first evidences of decay, 
reaching its full in fallen trunks, where the hard wood is 
wasting to powder. 

Scattered over the heavens are millions of stars, each 
representing a certain degree of development. The cloud 
forms of the nebulae tell us of stellar origins; the white, 
yellow, and red stars illustrate the rise and decline of stellar 
life; and the Earth itself affords a picture of what may 
remain after light and heat have been extinguished. 



CHAPTER II 

THE STUDENT OP THE NEW ASTRONOMY 

The traditional conception of the astronomer, while still 
applicable (with sundry limitations) in certain modern 
instances, does not accurately apply to the student of stellar 
evolution. According to the old view, the astronomer, soon 
after the setting of the Sun, retires to a lofty tower, from 
whose summit he gazes at the heavens throughout the 
long watches of the night. His eye, fixed to the end of a 
telescope tube, perceives wonders untold, while his mind 
sweeps with his vision through the very confines of the uni- 
verse. The lineal descendant of the seers and soothsayers 
of the Chaldeans, he dwells apart, finding little of interest in 
the ordinary concerns of the world, so occupied are his 
thoughts with celestial mysteries. 

Now there can be no doubt that the study of stellar evo- 
lution brings a degree of pleasure and enthusiasm which it 
would be difficult to surpass. The joys of the pioneer, the 
excitement that comes to him who looks for the first time 
upon an unknown land, the intense satisfaction of discovery, 
all belong to the successful investigator. Moreover, mere 
gazing through a telescope, as distinguished from the pains- 
taking work of modern astronomers with micrometer or 
photographic plate, is still competent to reveal new or chan- 
ging phenomena, and important discoveries are yet to come 
to the alert and careful observer. It is pleasant to picture 
the surprise and delight of Galileo when he first perceived 
spots on the supposedly immaculate surface of the Sun. His 
little instrument, much less perfect than a modern spy-glass, 
could reveal none of that intricate structure and exquisite 

9 



10 Stellar Evolution 

detail that are at once the joy and the despair of present-day 
snn-spot observers. But he had discovered a new and 
important fact, the basic principle of the science of astro- 
physics: he had shown that with suitable optical aid the 
physical structure of the heavenly bodies might be investi- 
gated. Prior to this time astronomy had concerned itself 
only with the positions and motions of the stars; now it 
became evident that each of these luminaries might present 
peculiar and distinguishing phenomena worthy of the most 
searching investigation. Discovery followed discovery in 
rapid sequence. The mottled face of the Moon, formerly 
without meaning, was suddenly revealed in unsuspected 
landscapes of valley, plain, and mountain, resembling, in 
curious degree, the variegated surface of the Earth. Jupiter, 
who had seemed to travel alone through the heavens, was 
found to possess four companions, whose revolutions about 
him forcibly suggested the revolutions of the planets about 
the Sun. The mysterious ansae, inclosing between them 
the globe of Saturn, were soon made out to be the more 
conspicuous elements of a vast incircling ring, unlike any- 
thing of earlier experience. With the growth of the tele- 
scope more marvels were brought to light, until it seemed, in 
sound reason, as though the universe would never cease to 
yield new knowledge to the explorer of its boundless wastes. 
^ Thus was established that conception of the astronomer 
that still persists, long after a new astronomy has come into 
being. Gazing through a telescope, as has been said, is still 
competent to bring discoveries ; for change is the very essence 
of celestial phenomena, and persistent watching must detect 
important facts, on which broad generalizations may be 
founded. But the eye and the telescope have been supple- 
mented by various instrumental aids which, in their multi- 
plication, have transformed the occupations of the astronomer. 
The micrometer, in its application to the accurate measure- 



The Student of the New Astronomy 11 

ment of place and form, permits changes to be detected which 
are beyond the perception of the eye. The photometer, in 
its precise determinations of brightness, has shown that stars 
whose light never varies are rather the exception than the rule. 
The photographic plate, used in conjunction with the tele- 
scope, has proved itself to be more sensitive than the human 
retina, in that it is capable of adding up into a visible record 
the invisible radiations received during an exposure of many 
hours. Finally, to mention but one more of the telescope's 
new adjuncts, the spectroscope has introduced a new and 
revolutionary principle into astronomy, permitting the chemi- 
cal and physical analysis of the most distant stars. 

Hence it is that the present-day student of astrophysics 
does not correspond to the traditional idea of the astronomer. 
His work at the telescope is largely confined to such tasks as 
keeping a star at the precise intersection of two cross-hairs, 
or on the narrow^ slit of a spectrograph, in order that stars 
and nebulae, or their spectra, may be sharply recorded upon 
the photographic plate. His most interesting work is done, 
and most of his discoveries are made, when the plates have 
been developed, and are subjected to long study and measure- 
ment under the microscope. His problems of devising new 
methods of calculation or reduction are as fascinating as the 
invention of new instruments of observation. Much of his 
time may be spent in the laboratory, imitating, with the means 
placed at his disposal by the physicist and chemist, the vari- 
ous conditions of temperature and pressure encountered in 
the stars, and watching the behavior of metals and gases in 
these uncommon environments. If, in the conviction that new 
and promising means of research are always awaiting applica- 
tion, he would advance into still unoccupied fields, he must 
devote himself to the design and construction of new instru- 
ments, to supplement the old. Kept thus in touch with the 
newest phases of physical and chemical investigation, the 



12 Stellae Evolution 

countless applications of electricity, the methods of modern 
engineering, and the practical details of workshop practice, 
his interest in these things of the world is likely to be quite 
as broad as that of the average man. His sympathy w^ith 
research in every branch of science must increase and 
strengthen as his conception of the great problem of evolu- 
tion is developed by his own investigations of its earliest 
phases. And the pleasure and enthusiasm derived from his 
studies must become, not like the vague passion of the mys- 
tic, whose inability to see clearly leads him to pursue strange 
gods, but such as every successful searcher after truth must 
experience, whether he deal with the vast dimensions and 
distances of the heavenly bodies, or with the minute but no 
less marvelous phenomena of microscopic life and form. 

Now, w^hile it cannot be too strongly emphasized that the 
student of stellar evolution can have no sympathy with the 
mystic, whose habit of thought must be the very antithesis 
of his own, yet it is true that the imagination, when properly 
exercised and controlled, is to be regarded as his best aid to 
progress. The question of control is so important that it 
may well be mentioned first. For nothing has done more 
injury to science than the play of imaginations subject to no 
control, on the part of men who enjoy in the public press the 
rank of scientific authorities. Thus great sun-spots become 
the innocent cause of earthquakes or tornadoes, not to speak 
of their effect upon the price of wheat. Comets, once the 
unerring portents of war and pestilence, still carry the brands 
of conflagration, and threaten at each apparition to destroy 
the Earth. Mystic properties are ascribed to the center of 
the universe, and a well-known planet, because it is incor- 
rectly assumed to be stationed there, is dogmatically asserted 
to be the only possible abode of human life. There is a fine 
field here for humor and amusing speculation, as the author 
of the "Moon Hoax," and other more recent writers, have 



The Student of the New Astronomy 18 

shown us. But humor is not always intended: the pronun- 
ciamentos go forth in the name of science, and are so accepted 
by a host of intelligent persons, who naturally believe that the 
supposed authorities have reached their conclusions by scien- 
tific methods. Thus there arises a false conception of science, 
and a popular demand for wonders, which is not easily 
satisfied by acquaintance with the less sensational facts. 

But though dangerous when unrestrained, the imagina- 
tion, when rightly exercised, is the best guide of the 
astronomer. His dreams run far ahead of his accomplish- 
ments, and his work of today is part of the development of 
a plan projected years ago. He perceives that only a few 
generations hence many of the instruments and methods of 
his time are to be replaced by better ones, and he strains his 
vision to obtain some glimpse, imperfect though it be, into 
the obscurities of the future. As he sits in his laboratory, 
surrounded by lenses and prisms, gratings and mirrors, and 
the other elementary apparatus of a science that subsists on 
light, he cannot fail to entertain the alluring thought that 
the intelligent recognition of some well-known principle of 
optics might sufiice to construct, from these very elements, 
new instruments of enormous power. He learns of some 
advance in engineering or in the art of the glass-maker, and 
dreams of new possibilities in its application to the construc- 
tion of his telescopes or the equipment of his laboratory. He 
reads of discoveries in physics or chemistry, and at once his 
mind is busy in its endeavor to apply the new knowledge to 
the solution of long-standing cosmical problems. 

But here, again, we see the need of control ; for with such 
a multiplicity of interests, and such constant stimulus to the 
imagination, the danger of mere dilettantism is obvious. 
With scores of problems suggesting themselves for solution, 
and with attractions on every hand, each rivaling the other 
in its apparent possibilities of development, the chief difficulty 



14 Stellar Evolution 

is to choose wisely. It is not a question of searching for 
something to do, but of picking out those things which are 
most worthy of pursuit. Here the importance of having a 
definite and logical plan of research becomes apparent. Such 
a plan may involve a single investigation, continued along 
systematic lines over a long period of years, or it may com- 
prise several investigations, carried on simultaneously. In 
a large observatory each piece of work acquires increased 
importance if it is selected, not at random, or solely because 
of its intrinsic value, but rather because of the part it plays 
in a single logical scheme of research. Its intrinsic impor- 
tance need not be in the least diminished by its relationship 
to other work, while the illumination which its results cast on 
the other investigations of the scheme can hardly fail to 
improve them, and may even reveal the chief source of their 
meaning. Moreover, the same research, if carried on else- 
where, might prove of small value, in the absence of such 
suggestions and modifications as are sure to come from the 
related investigations. We shall have occasion to revert to 
this question in discussing a plan of attack on the general 
problem of stellar evolution. 



CHAPTEK III 
THE SUN AS A TYPICAL STAR 

Befoee proceeding to the more detailed portions of our 
discussion, let us examine the present condition of the bodies 
with which we are to deal, and briefly trace out those ele- 
ments of relationship which it will be our purpose later to 
describe more fully. Let us begin with the consideration of 
a single object, which we may afterward compare with other 
objects less easily observed because of their greater distance 
from the Earth. 

The photographic reproduction in Plate II represents the 
Sun, as seen with an ordinary telescope. So far as could be 
judged from this picture, the Sun might be described as a 
luminous sphere, brighter in its central part than near its 
circumference, and marked with dark spots, irregularly dis- 
tributed over the surface. On closer examination it will also 
be seen that there are certain bright regions, which are most 
easily noticed near the edge of the Sun. The dark spots are 
the well-known sun-spots, first discovered by Galileo, while 
the bright regions are the faculae, which have also been 
known since the invention of the telescope. At times of total 
eclipse, when the bright body of the Sun is covered by the 
dark body of the Moon, shielding our atmosphere from the 
usual brilliant illumination, red flames, sometimes reaching 
heights of several hundred thousand miles, may be seen rising 
from a continuous sea of flame, which completely incircles 
the Sun. These are the prominences, and the continuous 
mass of flame from which they rise is the chromosphere 
(Plate III).^ Extending far beyond these flames into space, 

1 See the remarks on anomalous dispersion, p. 148. 

15 



16 Stellar Evolution 

sometimes to a distance of millions of miles, is the corona, 
which shines with a silvery luster somewhat inferior in bright- 
ness to that of the full Moon (Fig. 2, Plate IV). 

An analysis of the light of the Sun, made with the spectro- 
scope, has shown the presence of the vapors of iron, sodium, 
magnesium, calcium, hydrogen, and many other substances 
known to us on the Earth. In fact, it has been remarked 
that if the Earth were heated to the temperature of the Sun, 
the light emitted by its vapors would resemble closely, when 
analyzed with the spectroscope, the light emitted by the Sun. 
Thus the chemical composition of the Earth and the Sun is 
much the same, although we have evidence of the existence 
in the Sun of a large number of substances not yet found on 
the Earth. This same means of analysis has led to the dis- 
covery that the chromosphere, and the prominences which 
rise out of it, are composed of the vapor of calcium and of the 
light gases helium and hydrogen. The sun-spots, too, have 
also been found to have a characteristic chemical compo- 
sition; while the corona emits rays which probably indicate 
the presence in it of very light and tenuous gases. 

Observations of the Sun, continued without interruption 
for more than half a century, have shown that the spots are 
not constant in number, but vary in a characteristic way in 
a period of about eleven years. At times of sun-spot maxi- 
mum the surface of the Sun is marked by large numbers 
of spots, which are found on attentive observation to be the 
scene of great activity, and frequently the source of the most 
violent eruptions. At this period the prominences are large 
and abundant, and testify to the general condition of disturb- 
ance by exhibiting, from time to time, eruptive phenomena 
on a very large scale, in which great masses of gas have 
been known to shoot upward with velocities of hundreds of 
miles a second. With the passage of time these evidences of 
disturbance and activity become less and less marked, until 



The Sun as a Typical Stae 17 

finally, during the minimum period, the surface of the Sun 
for months together may be wholly devoid of sun-spots. 
The prominences also become less numerous, and eruptive 
phenomena, so common during the maximum period, are 
rarely to be observed at the minimum. Even the corona 
undergoes changes in form which are perfectly charac- 
teristic, and show a definite connection with the sun-spot 
period. 

So much for the Sun and its more conspicuous phenomena. 
We are now led to inquire whether it has any counterparts 
among the other heavenly bodies. Let us suppose the Sun 
removed to the distance of the nearest fixed stars. Its light 
would then be reduced in so great a degree as to be sur- 
passed by that of many of the brighter stars, though it 
would still remain one of the more conspicuous objects in 
the heavens. The planets of the solar system would be 
wholly beyond the range of observation, even with the most 
powerful telescopes. The light of the Sun would appear 
yellowish, and it would be impossible to distinguish it from 
certain stars which also shine with a yellowish light. Spec- 
troscopic analysis of the light of these stars reveals the 
presence in their atmospheres of elements familiar to us on 
the Earth; indeed, the chemical composition of some of 
them can be shown to be practically identical with that of the 
Sun. On account of its immense distance, the Sun's disk 
would be reduced to a minute point of light, as in the case of 
the other stars, and the sun-spots, prominences, corona, and 
other phenomena would be wholly invisible. For the same 
reason, such phenomena, though undoubtedly present in 
other stars, are hidden from observation. We may there- 
fore conclude that the Sun is a star, practically identical in 
chemical composition and in physical constitution with many 
other stars in the heavens, and ranking in size below many 
of these objects. 



18 . Stellae Evolution 



A very casual acquaintance with the stars, based upon 
naked-eye observations, is sufficient to make one familiar 
with the fact that they differ from each other as much in 
color as they do in brightness. Such objects as Sirius shine 
with a bluish-white light, whereas Arcturus is yellowish like 
the Sun. Antares, in the Scorpion^ is a fine example of a 
red star, and with the telescope smaller stars may be seen 
of a deeper red color. Spectroscopic study of these various 
classes of stars shows in the clearest way definitive peculiari- 
ties, which may form the basis of a system of classification. 
Indeed, we apparently find ourselves in the presence of stars 
in every stage of growth, from the earliest, as represented 
by the bluish-white objects, to the latest, typified by the red 
stars^ (Fig. 1, Plate IV). Intermediate in point of develop- 
ment are yellowish stars like the Sun. 

In various parts of the heavens clusters may be observed, 
in some of which the stars are widely scattered, as in the 
Pleiades, while in others they are densely packed together, 
so closely that several thousand stars may sometimes be seen 
within an area so small that to the naked eye they appear like 
a single hazy star. Since we find clusters of every degree of 
density, and since the stars in the heart of some of these 
clusters are too close together to be separated by the tele- 
scope, the question long ago arose whether the nebulae, 
which seem to resemble luminous clouds in the heavens, are 
to be regarded as star clusters so dense as to be beyond 
telescopic resolution. It was not until the spectroscope had 
been applied by Huggins (see p. 54) that this question was 
finally settled. It then appeared that some of the nebulae, at 
least, are vast masses of luminous gas, and that they are 
therefore not composed of separate stars. It might then be 
inquired what part in the scheme of evolution such nebulae 
play. It will be shown in the course of this book that there 

1 See the cautionary remarks on p. 198. 



The Sun as a Typical Star 



exists between stars and nebulae a relationship so intimate 
as to leave little doubt that stars are condensed out of nebulae 
through the long-continued action of gravitation. It thus 
seems probable that the nebulae represent the stuff from 
which stars are made, in its primitive and uncondensed 
state. 



CHAPTER IV 
LARGE AND SMALL TELESCOPES 

It must soon appear, to one who seeks in the heavens 
with unaided vision for evidences of stellar evolution, that 
but little progress can be made without powerful instrumental 
means. When the nature of the problem is considered, and 
it is remembered that all observations of the stars must be 
made from the surface of a minute body moving through the 
midst of the universe, the only cause for surprise will be that 
instruments of sufficient power for our purpose can be con- 
structed. The distances of the stars are so enormous that it 
might seem hopeless ever to solve the problem of their phys- 
ical constitution, or to analyze them as the chemist resolves 
into its elements a substance in his laboratory. 

Let us consider what must be accomplished before we may 
even begin to study the subject of stellar evolution. In the 
course of our work we must deal with stars which are not only 
invisible to the naked eye, but are beyond the reach of any 
except the most powerful telescopes. We must find the means 
of collecting the light from such bodies, not only those rays 
which, if intense enough, could be seen by the eye, but 
also those which, because of the structure of the eye, are 
wholly invisible. After collecting together such rays, we 
must subject them to analysis by instruments which will per- 
mit us to draw conclusions, both as to the nature of the 
chemical elements present in the star's atmosphere and as to 
the physical condition of these elements, illustrated by the 
pressure and the temperature to which they are subjected. 
Although we may never hope to see a star's actual disk, 
even in the most powerful telescopes of the future, as other 

20 



Large and Small Telescopes 21 

than a minute point of light, we must find means of differ- 
entiating one part of the star from another and of determin- 
ing, for example, whether the vapor of carbon lies above 
or below that of iron or sodium in its atmosphere. If 
luminous clouds, like those on the Sun, are strikingly char- 
acteristic of the star under observation, we must be able to 
detect their presence, though we may never see their form. 
If, as in the case of temporary stars, vast temperatures or 
pressures may produce great differences in physical condition 
between the inner and outer parts of a stellar atmosphere, 
w^e must learn a way of discovering such dift'erences and of 
ascribing them to their true cause. Incidentally, and as a 
necessary precedent to these studies, we must be able to 
determine whether the star is moving toward or away from 
the Earth, and to measure its velocity in either direction with 
great precision. 

Moreover, our means of analysis must be so refined that 
they shall enable us to investigate, not merely the general 
physical and chemical properties of single stars, but also 
those minute peculiarities of composition or of motion which 
may relate them to other stars, and define their precise place 
in some general scheme of stellar evolution. We must have 
some means at hand which will bring to light the forms of 
nebulae, even though they be invisible to a trained eye aided 
by the most powerful telescope ever constructed. Being 
given these forms, we must seek for evidences of relationship 
between the cloudlike nebulae and the stellar points which 
they surround. And the means of analysis which tells us of 
the constitution of the stars must also tell us of the nature of 
the nebulae, thus serving to establish relationships with stars 
which no mere indications of position or of structure could 
provide. 

A refracting telescope consists of a lens (object-glass) 
usually mounted at the end of a long tube, which is pointed 



22 Stellar Evolution 

at the object to be observed. In tlie present case we will 
suppose this to be the Moon. The lens forms an image of 
the Moon at the lower end of the tube, just as the lens of a 
camera forms an image on the ground-glass. Indeed, a tele- 
scope may be regarded as nothing more or less than a long 
camera, in which a tube is substituted for the ordinary bellows. 
By putting a plate at the point where the image is formed, and 
giving a suitable exposure, the Moon may be photographed, 
just as a landscape is photographed with the camera. For 
eye observations, however, the image formed by the telescope 
is looked at through a small lens called an eye-piece. The 
image is magnified in the same way, and to the same extent, 
as any object would be if looked at with the eye-piece, used 
as an ordinary hand magnifier. The total magnifying power 
of the telescope, however, of course depends not only upon 
the magnifying power of the eye-piece, but also upon the size 
of the image formed by the object-glass. The size of this 
image is determined solely by the focal length, or distance 
from the object-glass to the image. Suppose, for example, 
we have two telescopes, with object-glasses of the same diam- 
eter, but of different focal lengths. The one of longer focal 
length will give the larger image. If the focal length is twice 
that of the other telescope, the image will be twice as large. 
With the same eye-piece, therefore, the magnifying power of 
the longer telescope will be twice that of the shorter one. 

We thus see that the size of the image given by a tele- 
scope does not depend upon the diameter of its object-glass. 
The brightness of the image, however, evidently does depend 
upon the amount of light concentrated in it, and this increases 
with the diameter of the object-glass. If we double the 
diameter of the object-glass, we get four times as much light 
in the image of a star; for the amount of light collected 
depends upon the area of the object-glass, and this increases 
as the square of its diameter. 



Large and Small Telescopes 23 

These details are worth remembering, for they determine, 
in great measure, the relative advantages of large and small 
telescopes. There is another consideration, however, of the 
first importance, which must not be overlooked. A small 
telescope is limited, by the very nature of light, in its power 
of separating two closely adjacent stars. If these stars are 
less than a certain distance apart, no increase in the magnify- 
ing power of the telescope, either through increase in its focal 
length or through the use of a more powerful eye-piece, can 
possibly show them as separate objects. The reason lies in 
the fact that the image of a star in a telescope is a minute 
disk, the diameter of the disk depending on the size of the 
object-glass. The disk grows smaller as the object-glass 
grows larger; so it is easy to see why a large telescope will 
divide a close double star when a small one will not: the 
star images, which are of sensible diameter and consequently 
overlap, as seen in the small telescope, are reduced by the 
high resolving power of the large telescope to such minute 
dimensions that they appear distinct and separate. 

Here, perhaps, a word of explanation may be useful; for 
it is not at first sight obvious that a star should appear 
smaller' in a large telescope than in a small one. Such a 
statement would not be true of the Sun, Moon, or planets. 
These objects are all comparatively near the Earth, and even 
a moderate magnifying power will show them (except the 
most distant planets) as disks on which structural details are 
visible. The stars, however, are so inconceivably remote 
that no telescope, however powerful, can show their true 
disks. They are mere points of light, brighter, and for this 
reason apparently larger, in the case of the brilliant stars, but 
always becoming more minute and pointlike under the most 
favorable atmospheric conditions and with the most powerful 
instruments. 

The spurious disks, which would have no existence if 



24 Stellak Evolution 

light-waves were infinitely short, appear large in small tele- 
scopes, but small in large ones. In the Yerkes telescope, for 
example, stars separated by only a tenth of a second of arc 
can be resolved under the best atmospheric conditions. A 
four-inch telescope cannot separate stars that are less than a 
second of arc apart, no matter what magnifying power be 
applied. In such an instrument, therefore, the thousands of 
double stars whose components are separated by less than a 
second appear as single stars. In the same way, minute 
markings, lying close together on the Sun, Moon, or planets, 
are not separately distinguished in a small telescope, while 
in a large one they may be seen as distinct objects, provided 
the atmospheric conditions are sufficiently favorable. 

We may sum up the preceding remarks by saying that in 
all astronomical observations which involve the separate and 
distinct recognition of very closely adjacent stars, or a knowl- 
edge of the most minute structure of the Sun, Moon, or 
planets, large telescopes must be employed under excellent 
atmospheric conditions. Furthermore, if it is a question of 
collecting sufficient light, either for eye observations, or for 
photography, or for spectroscopic analysis, from an extremely 
faint star, the great area of a large object-glass or mirror also 
becomes essential. Nevertheless it will be shown that for 
many important investigations small telescopes are equal or 
even superior to large ones. 

This brings us to the much-discussed question of the 
relative advantages of large and small telescopes, regarding 
which a great deal has been written. On the one hand, we 
hear the amusing claims of the promoters of the great tele- 
scope which was to be the clou of the last Paris Exposition. 
This immense instrument — which does not seem to have been 
completed, and is now lying unused — was to bring the Moon 
within the observer's grasp — if he could reach a meter! 
The light-heartedness of this claim is manifest when it is 



Large and Small Telescopes 25 

remembered that no existing telescope, under the best 
atmospheric conditions, has ever shown the Moon as well as 
it would appear to the unaided eye at a distance of tifty 
miles. 

On the other hand, it has been stated, with great insist- 
ence, that it is absurd to use a telescope of more than four 
inches' aperture east of the Mississippi River, or of more than 
six inches' aperture in the better atmospheric conditions 
west of it. This statement, although not so extreme as the 
one which emanated from Paris, is entirely misleading and 
unwarranted by the facts. It was probably intended to 
emphasize a conviction that the atmospheric conditions in 
the eastern part of the United States are very bad, and un- 
suited for large telescopes. Now, it is quite true that atmos- 
pheric disturbances are the bane of astronomers in all parts 
of the world ; we shall have occasion to discuss this question 
in a future chapter. It is also true that the meteorological 
conditions are, on the average, much more favorable for 
astronomical observations in the southwestern part of the 
United States than east of the Mississippi River. But it 
cannot be denied that many of the valuable observations 
turned out by our eastern observatories are directly due to 
the fact that they are equipped with large telescopes. That 
these telescopes would do more and better work under better 
conditions goes without saying. Most of them would not 
exist at all, however, if it had been a question of establishing 
them some thousands of miles from the universities or col- 
leges with which they are connected. 

To those who have used both large and small telescopes, 
the great advantages of large instruments for many kinds of 
work are well known. I have heard a European astronomer 
exclaim, when looking at Jupiter for the first time with the 
forty-inch Yerkes telescope, that his years of study of this 
planet with a small telescope seemed almost useless, so much 



26 Stellar Evolution 

more of detail could he perceive at a single glance. I have 
seen minute structure on the Moon with this telescope, no 
trace of which could be made out with a twelve-inch tele- 
scope on the same evening. Countless fine bright lines in 
the spectrum of the chromosphere, which could never be 
detected with the twelve-inch, are easily seen with the 
forty-inch. Burnham has separated double stars at the 
theoretical limit of resolution of the Yerkes telescope, and 
Barnard has observed the tiny fifth satellite of Jupitey^ when 
it was beyond the reach of all but the largest existing instru- 
ments. When I think of these observations and of Ritchey's 
photographs of the Moon and star clusters, Frost's and 
Adams' photographs of the spectra of faint stars, and the no 
less important results obtained by Schlesinger, Parkhurst, 
Ellerman, Fox, and others with the Yerkes telescope; and 
when I remember that most of these observations or results 
could not have been obtained with a small telescope, I see no 
possible reason for denying the manifold advantages of large 
instruments. My illustrations have been confined to obser- 
vations made with the Yerkes telescope, because of personal 
knowledge of them. But they could be greatly multiplied if 
the remarkable work of the Lick telescope and of other large 
instruments were drawn upon for examples. In the next 
chapter, through the aid of photography, some of the relative 
advantages of large and small telescopes will be illustrated. 



CHAPTER V 
ASTRONOMICAL PHOTOGRAPHY WITH CAMERA LENSES 

The emphasis laid in the last chapter on the importance 
of large telescopes must not be supposed to mean that small 
telescopes are of little value. The single fact that Burnham 
discovered 451 new double stars with a six-inch refractor 
(Plate V) is sufficient evidence to the contrary. It is quite 
true that small telescopes are not well adapted for certain 
classes of work, in which large telescopes excel. But their 
superiority over large telescopes is no less evident in other 
fields. The equipment of an observatory recognizes this by 
the provision of both large and small telescopes, each designed 
for use in the investigations for which it is particularly suited. 
In fact, the characteristic of a modern astrophysical observa- 
tory which distinguishes it most clearly from the old observa- 
tory of one or two instruments is the careful adaptation of a 
multiplicity of special apparatus to certain narrowly defined 
purposes. The day of the universal instrument has passed, 
for conditions similar to those which have resulted in the 
development of the innumerable special tools of the modern 
machine shop obtain also in the observatory. 

The amateur astronomer should keep this fact clearly in 
mind. There is some reason to fear that the large and 
expensive equipments of modern observatories have tended 
to discourage the worker with small instruments. As one 
who has looked at the subject " from both sides, I may be 
permitted to oppose this pessimistic view. Far from believ- 
ing that recent developments have been detrimental to the 
amateur, I am strongly of the opinion that his opportunities 
for useful work have never before been so numerous. The 

27 



28 Stellae Evolution 

importance of this subject, due to the high value of the 
contributions to astronomy made by amateurs in the past, 
has led me to devote a subsequent chapter to opportunities 
for work with inexpensive instruments. 

In considering the peculiar advantages of small telescopes 
in certain fields of research, attention must be called at the 
outset to the important part played by photography in the 
astrophysical work of the present day. The photographic 
plate, through its power of storing up impressions made by 
feebly luminous rays, has in most cases an immense advan- 
tage over the eye. The eye perceives almost at once as much 
as can be seen by long gazing at a faint object. But the 
photographic plate continues, hour after hour, and perhaps 
night after night, to accumulate impressions, so that with 
sufficiently long exposures, objects far too faint to be seen 
by the eye with the same telescope are clearly and per- 
manently recorded. Moreover, the photographic plate is 
sensitive to light-waves which are too short to produce the 
least effect upon the eye, and in this power of recording 
objects which otherwise could never be rendered visible, no 
matter what their intensity of radiation, the plate presents a 
second great advantage. Because of these and other points 
of superiority, which far outweigh certain slight defects that 
in some few instances still leave the plate inferior to the eye, 
the photographic method is now exclusively employed for 
many kinds of observations. 

Some of the most important results of astronomy have 
been derived from the use of an ordinary camera, having 
just such a lens as is found in the possession of thousands 
of amateur photographers. If we take an ordinary camera 
and point it on a clear night toward the north pole, it will 
be found, after an exposure of one or two hours, that the 
stars which surround the pole have drawn arcs of circles 
upon the plate (Plate VI). This is due to the fact that 



Astronomical Photography 29 

the Earth is rotating upon its axis at such a rate as to cause 
every star in the sky to appear to travel through a complete 
circle once in twenty-four hours. Since the pole is the place 
in the sky toward which the Earth's axis is pointing, it is 
easy to understand that the nearer the star lies to the pole, 
the smaller does this circle become. As we move away from 
the pole we find the curvature of the star trails growing less 
and less, until at the equator they appear as straight lines. 

Just such photographs as these are frequently employed 
in astrophysical investigations; e. g., for the purpose of 
recording variations in a star's brightness, which would be 
shown on the plate by changes in the strength of the trail. 
But for most purposes it is desirable to have photographs of 
stars in which they are represented as points of light rather 
than as lines. To obtain these photographs it is necessary to 
mount the camera in such a way that it can be turned about 
an axis parallel to the Earth's axis, at a perfectly uniform 
rate, once in twenty -four hours. A camera so mounted 
becomes an equatorial photographic telescope, differing in no 
important respect, save in the construction of its lens, from 
the largest refractors. 

Here, for example, is a photograph (Plate VII) of the 
Bruce photographic telescope of the Yerkes Observatory. 
This instrument has a compound lens ten inches in diameter, 
made by Brashear from four lenses suitably combined, of 
such curvature as to form an image at a point only fifty 
inches distant from the optical center of the lens system. It 
will be seen that such a lens must produce a very bright and 
highly concentrated image, in which the various objects are 
crowded close together because of the small scale of the 
picture. If the same lens were so constructed as to form an 
image ten times as far distant from the photographic plate, 
the several elements of the picture would then be ten times 
more widely separated, and a longer time would be required 



30 Stellar Evolution 

to photograph them, on account of the spreading of the same 
amount of light over a larger surface. As will be seen from 
the illustration, the tube which carries the lens and photo- 
graphic plate is mounted in such a way that it may be turned 
about an axis parallel to the axis of the Earth by means of a 
driving-clock, placed in the upper part of the iron supporting 
column. The same mounting carries not only the ten-inch 
lens, but also the lens of a guiding telescope, through which 
the observer watches a star during the entire period of 
exposure, continued, perhaps, for many hours. He may 
thus correct any slight irregularity in the motion of the tele- 
scope by certain screws provided for the purpose, which per- 
mit him to keep the star accurately at the intersection of two 
illuminated cross- wires. The driving of the clock is so accu- 
rate that this is accomplished almost automatically, though 
small changes in atmospheric refraction and other causes 
require minute displacements of the instrument to be made 
from time to time, to insure the perfect immobility of the 
stellar images upon the photographic plate. 

Besides the ten-inch camera and the guiding telescope, 
the Bruce telescope carries three other cameras, with lenses 
of 6 inches, 3.4 inches, and 1.6 inches aperture respectively. 
Thus four photographs of the same part of the heavens, on 
different scales, determined by the focal lengths of the 
lenses, are obtained in a single operation. Our knowledge 
of the structure of the vast girdle of stars that forms the 
Milky Way is derived in very large part from a study of 
photographs made with such an instrument. At the Lick 
Observatory Barnard used the six-inch Willard lens to great 
advantage in photographing these star clouds, and of late, 
through the opportunity afforded by the Hooker Expedition 
at the lower latitude of Mount Wilson, he has carried his 
work farther south of the celestial equator. The Bruce tele- 
scope, temporarily transferred from the Yerkes Observatory to 



Astronomical Photography 31 

Mount Wilson for use during the spring and summer of 1905, 
has yielded some remarkably fine results in Barnard's hands. 
The smallest of the four photographs made in a single opera- 
tion is taken with an ordinary "magic-lantern" lens of 1.6 
inches aperture and 6.4 inches focal length. This shows a 
a region about fifteen degrees^ across within a circular area 
about 1.7 inches in diameter on the photographic plate. 
With the ten-inch lens the field of sharply defined images is 
limited to about eight degrees, but it is still large enough 
to include extensive star clouds and nebulae. The larger 
scale, due to the greater focal length of the ten-inch lens, 
brings out details of structure that are not visible on the 
smaller photographs. Plates VIII and IX, reduced from the 
originals in the same proportion, illustrate the relative scales 
of the photographs made with the two lenses. 

The Milky Way, as revealed by such photographs, is 
a most extraordinary spectacle. The countless stars that 
compose it are grouped in every conceivable manner, and 
intertwined with long reaches of diffuse nebulous clouds. 
Here and there vacant regions, sometimes apparently darker 
than the background of the heavens, resemble vast lanes, 
extending through the entire thickness of the star clouds, or 
perhaps lead one to suspect that an obscuring medium may 
be cutting off the light from immeasurably distant bodies. 
Again, a nebula of great extent, diffuse on one side and sharply 
bounded on the other, may suggest the action of forces be- 
yond our present means of investigation , The filmy veils 
spread by certain nebulae seem to envelop the stars in mist, 
though in most cases we cannot say with certainty whether 
the stars are actually within the clouds, or remote from them 
in the line of vision. The surest test of relationship between 
stars and surrounding nebulae is afforded by the spectroscope, 

1 Readers who are not accustomed to angular measure may be reminded that 
the two " pointers " of the " Dipper " are about five degrees apart. 



32 Stellae Evolution 

as will be shown in a subsequent chapter. It has been found 
that stars of different spectral types, which are ordinarily 
assumed to indicate different degrees of development, are 
not equally represented in the Milky Way. The connection 
between these stars and surrounding nebulae, and the possible 
relationship between spectral type and the grouping of the 
stars in the cloudlike forms of the Galaxy, is one of the 
important problems of the present time. Our knowledge of 
the Milky Way and its structure is still very meager, but the 
future is certain to bring great advances. 

These illustrations may suffice to show the uses of the 
ordinary camera lens in investigations bearing upon the 
general structure of the Milky Way. A simple compari- 
son will serve to bring out both the advantages and dis- 
advantages of large telescopes in studies of a similar kind. 
Plate X shows the Milky Way in Ophiiicns from one of Bar- 
nard's photographs made with a portrait lens. It affords a 
superb picture of this part of the sky, such as no visual 
observations with any telescope could supply. If the same 
region of the heavens were examined with a large telescope, 
the field of view would be so restricted that no proper 
impression could be obtained as to the character of the 
Milky Way or the distribution of the stars within it. It 
would, of course, be possible to count one by one the hun- 
dreds of stars included within a single field of view, and by 
long and laborious measurements to map these stars and 
ultimately to build up, from combination into a single picture 
of the results thus obtained, a representation of the Milky 
Way. However, such a task would occupy years of labor, 
and the result would be less valuable, for many purposes, 
than that illustrated in Plate X. This picture is an auto- 
graphic record, showing not only the distribution of the 
stars, but also their relative brightness on the date of the 
exposure. 



Astronomical Photography 33 

Since such results are clue to photography, the comparative 
value of large telescopes should be judged by the same 
means. Plate XI is a reproduction of a photograph of the 
cluster Messier 11, which is represented in Plate VIII as a 
small circular white dot in the upper part of the picture. 
The short focal length of the camera lens, which causes it to 
form upon the plate a small-scale picture covering a large 
region in the heavens, is not competent to separate out the 
single stars of this cluster. The photograph reproduced in 
Plate XI was made by Ritchey with the forty -inch Yerkes 
telescope, which has a focal length of sixty-four feet, as com- 
pared with the focal length of 6.4 inches of the camera lens 
used for Barnard's photograph. The scale of the negative 
obtained with the Yerkes telescope is therefore about 120 
times as great as in the case of the camera lens. This 
great scale, while disadvantageous so far as it bears upon 
the question of the general structure of the Milky Way, 
would be in the highest degree advantageous if the problem 
under consideration involved the study of the individual stars 
in the cluster Messier 11. With the camera lens these stars 
are so close together upon the plate that their separate images 
are confused. With the Yerkes telescope the images are 
widely separated from one another, permitting the position 
and the brightness of each star to be determined with great 
precision. The Bruce lens gives an intermediate scale. If 
Plate IX had been enlarged in the same proportion as Plate 
XI, this cluster would be shown fairly well resolved. But 
Messier 13 (Plate XIX) is far beyond the capacity of the 
Bruce lens. 

It may be of interest to include here another photograph 
illustrative of the advantages of great focal length for certain 
classes of work. Plate XII represents a photograph of the 
Moon, made by Ritchey with the twelve-inch Kenwood tele- 
scope, which is eighteen feet long. This picture gives an 



34 Stellae Evolution 

excellent general idea of the lunar topography. But if the 
detailed structure of the lunar mountains is to be investigated, 
such a picture as that reproduced in Plate XIII would evi- 
dently be far more effective for the purpose. Theopliilus, the 
great ring mountain here represented, may be seen in Plate 
XII on a smaller scale. The large-scale picture was obtained 
by Ritchey with the forty-inch telescope, which, as already 
remarked, has a focal length of sixty-four feet: The scale 
of the original photograph was therefore about three and 
one-half times as great as that of the photograph taken with 
the Kenwood telescope. In consequence of the larger scale 
of the Yerkes picture, it brings out many small details which 
are entirely lacking on the Kenwood photograph. 

These illustrations of the separating power of the large 
telescope may lead us to an examination of the instrument 
itself (Plate XIV) . Although so much larger, it differs in no 
essential particulars from the Bruce photographic telescope, 
also made by the firm of Warner & Swasey. The great 
weight of the forty-inch lens, amounting with its cell to half 
a ton, requires that the tube which carries it shall be of 
immense rigidity and strength. This tube, sixty-four feet 
in length, is supported at its middle point by the declination 
axis, which in its turn is carried by the polar axis, adjusted 
to accurate parallelism with the axis of the Earth. By 
means of driving mechanism in the upper section of the iron 
column, the whole instrument is turned about this polar 
axis at such a rate that it would complete one revolution in 
twenty-four hours. Although the moving parts weigh over 
twenty tons, the telescope can be directed to any part of 
the sky by hand, but this operation is much facilitated by 
the use of electric motors provided for the purpose. When 
once directed toward the object to be observed, it will fre- 
quently happen that the lower end of the telescope is far 
out of reach above the observer's head. For this reason the 



ASTKONOMICAL PHOTOGRAPHY 35 

entire floor of the observing-room, seventy-five feet in diam- 
eter, is constructed like an electric elevator, which, by moving 
a lever, can be made to rise or fall through a distance of 
twenty -three feet. Thus the lower end of the telescope is 
rendered accessible even when the object is near the horizon 
(Plate XV). In order that the observing slit may be di- 
rected to any part of the sky, the dome, ninety feet in diameter 
(Plate XVI), is mounted on wheels and can be turned to any 
desired position by means of an electric motor controlled 
from the rising-floor. 

The telescope is used for a great variety of purposes in 
conjunction with appropriate instruments, which are attached 
to the lower end of the tube near the point where the image 
is formed. We have already examined a photograph of a star 
cluster taken with this telescope, but without describing the 
process of making it. As a matter of fact, the forty-inch 
object-glass was designed for visual observations, and its 
maker, the late Alvan G. Clark, had no idea that it would ever 
be employed for photography. Without dwelling upon the 
distinguishing features of visual and photographic lenses, it 
may be said that the former are so designed by the optician 
as to unite into an image those rays of light, particularly the 
yellow and the green, to which the eye is most sensitive. 
With the only varieties of optical glass obtainable in large 
pieces, it is impossible to unite into a single clearly defined 
image all of the red, the yellow, the green, the blue, and the 
violet rays that reach us from a star. Therefore, when the 
optician decides to produce an image most suitable for eye 
observations, he deliberately discards the blue and violet 
rays, simply because they are less important to the eye than 
the yellow and green rays. For this reason the image of a 
star produced by a large visual refracting telescope is sur- 
rounded by a blue halo, containing the rays discarded by 
the optician. These very rays, however, are the ones to which 



36 Stellar Evolution 

the ordinary photographic plate is raost sensitive; hence in 
a photographic telescope the blue and violet rays are united, 
while the yellow and green rays are discarded. 

The forty -inch telescope is of the first type, constructed 
primarily for visual observations. In order to adapt it for 
photography, Ritchey, then a member of the Yerkes Obser- 
vatory staff, simply placed before the (isochromatic) plate a 
thin screen of yellow glass, which cuts out the blue rays, but 
allows the yellow and green rays to pass. As isochromatic 
plates are sensitive to yellow and green light, there is no 
difficulty in securing an image with the rays which the 
object-glass unites into a perfect image. During the entire 
time of the exposure some star lying just outside the region 
to be photographed is observed through an eye-piece mag- 
nifying 1,000 diameters. This eye-piece is attached to the 
frame which carries the photographic plate, and is suscep- 
tible of motion in two directions at right angles to one an- 
other (Plate XVII). In the center of the eye-piece are two 
very fine cross-lines of spider web, illuminated by a small 
incandescent lamp. If the observer notices that through 
some slight irregularity in the motion of the telescope, or 
some change of refraction in the Earth's atmosphere, the 
star image is moving away from the point of intersection 
of the cross-lines, he instantly brings it back by one or both 
of the screws. As the plate moves with the eye-piece, it is 
evident that this method furnishes a means of keeping the 
star images exactly at the same position on the plate through- 
out the entire exposure. 

Many other comparisons of large and small telescopes 
might be given, and some of these will be included in sub- 
sequent chapters. They all serve to demonstrate that each 
telescope has advantages and disadvantages peculiar to its 
size and type of construction. For some purposes small 
camera lenses are to be preferred to all other instruments. 



ASTEONOMICAL PhOTOGEAPHY 37 

In fact, without their aid many investigations of the highest 
importance could never be undertaken. For other investiga- 
tions these short-focus instruments may be entirely unsuited, 
while refracting telescopes of great focal length may give 
excellent results. These larger telescopes also have their 
limitations, and must yield to reflecting telescopes in certain 
other kinds of work. The truth of this statement will be 
brought out in the next chapter. 



CHAPTER VI 

DEVELOPMENT OF THE REFLECTING TELESCOPE 

On a night in April, 1845, while sweeping the sky in 
the constellation of the Hunting Dogs, the observers with 
the great Parsonstown reflector discovered a spiral nebula. 
The instrument with which the discovery was made may well 
be regarded as one of the most remarkable scientific achieve- 
ments of the nineteenth century. With its immense mirror, 
six feet in diameter, having a focal length of fifty-four feet, 
the great telescope of Lord Rosse surpassed in size all others 
ever constructed. Unfortunately for the progress of science, 
the engineering methods of that day were inadequate to 
provide a suitable mounting for this gigantic instrument. 
All parts of the machinery had to be constructed on the spot, 
with such tools as the period and the circumstances afforded. 
It is no small credit to the Earl of Rosse that under these 
conditions the telescope was ever erected, and kept in active 
use by an able company of observers. Supported upon a ball- 
and-socket joint at its lower end, the enormous tube, swung 
in chains, was confined to observations within a short distance 
of the meridian by two flanking stone walls. The observer, 
mounted upon a platform far above the ground, saw the 
image of an object as he looked down into the tube. To 
the present-day astronomer, provided with every appliance 
to facilitate the finding of an object, and with an accurate 
driving-clock which moves the telescope so steadily and uni- 
formly as to maintain the image in the field of view for hours 
together, it is little short of marvelous that the observers 
with the great Parsonstown reflector were able to obtain 
results of value, ^ut, in spite of the difficulties to be over- 

38 



Development of Reflecting Telescope 39 

come, both in manipulating the telescope and in finding 
opportunities for observation under the cloudy skies of 
Ireland, Lord Rosse and his assistants recorded many val- 
uable discoveries in their memoirs. Of all these discoveries 
that of the spiral nebula in Canes Venatici was perhaps 
the most significant of the future (Plate LXXXVIII). Be- 
fore this chapter is concluded we shall see how this beautiful 
object, which once stood alone among the heavenly bodies 
as the only visible representative of a distinctly spiral form, 
has now come to be regarded, through the work of Keeler, as 
a type of the most interesting and the most numerous class 
of nebulae. 

The history of the Parsonstown reflector has in some de- 
gree resembled that of almost every reflecting telescope ever 
built. The infinite care expended by Herschel and by others 
who have followed him in the construction of mirrors for such 
instruments has been in large part annulled by the imper- 
fections of the mountings provided for the mirrors. In 
the period that preceded the introduction of photographic 
methods, these imperfections were far less serious than they 
would be considered from our present point of view. It is 
true that they hampered observation, and in the early days 
rendered accurate measurement with the telescope practically 
impossible. But the employment of the photographic plate 
has imposed a new condition, rigorous and unyielding, upon 
the constructors of telescope mountings. In order to secure 
satisfactory photographs, which shall do full justice to the 
optical qualities of the instrument, and show only such de- 
fects as atmospheric disturbances may produce, it is necessary 
that the mirrors be so rigidly supported, and so accurately 
moved by the driving-clock, that a stellar image shall not 
depart, during exposures of many hours, by so much as 
one-thousandth of an inch from a fixed position upon the 
photographic plate. 



4:0 Stellar Evolution 

In view of the difficulties to be overcome, it will be under- 
stood that to accomplish such a result is no small task. In 
the first place, the mirror, which is so sensitive to deformation 
that it will bend under its own weight unless supported by 
special apparatus, must be firmly mounted, yet without strain, 
at the lower end of an open tube. In the second place, pro- 
tection must be provided against currents of warm and 
cold air, and even against the heat radiated from the 
observer's body, on account of the great sensitiveness of the 
mirror to heat, and of the light-rays to irregular refraction in 
the telescope tube. These precautions having been taken, 
the tube must be so mounted that it can be moved with per- 
fect steadiness and uniformity about an axis parallel to the 
axis of the Earth. This condition is imposed by the neces- 
sity of counteracting the apparent motion of the star through 
the heavens, due to the rotation of the Earth. But while 
this rotation is uniform, the motion of the star is not, since 
the displacement of its apparent position from its true posi- 
tion in the heavens, due to the bending of its rays during 
transmission through the Earth's atmosphere, varies with the 
height of the star above the horizon. It therefore becomes 
necessary, as previously explained, to supplement the uniform 
motion of the driving-clock by corrections, accomplished by 
the hands of an observer. All these obstacles having been 
surmounted, there still remain serious sources of difficulty 
in the shaking of the telescope by the wind, the changes of 
temperature during the exposure, which alter the focal length 
of the mirror, and finally, most serious of all, disturbances 
in the atmosphere which tend to blur and confuse the image, 
instead of leaving it, sharp and well defined, to make its 
record upon the photographic plate. It should also be 
remembered that the observer must be prepared to hold his 
eye at the eye-piece, and correct every few seconds the posi- 
tion of the plate, throughout exposures lasting several hours. 



Development of Reflecting Telescope 41 

in an open dome where the temperature may not infrequently 
be below zero. 

After the erection of Lord Rosse's great reflector, the 
attention of opticians was confined mainly to the construction 
of refracting telescopes, which grew rapidly in size, reaching 
apertures of fifteen inches in the Harvard refractor (184:5), 
thirty-six inches in the Lick refractor (1889), and forty inches 
in the Yerkes refractor (1897). In these instruments care- 
ful attention was given to all details of mechanical construc- 
tion, and the Lick and Yerkes telescopes are among the most 
successful products of modern engineering. 

The development of the reflecting telescope has been due 
mainly to amateurs, whereas refractors have been made by 
professional opticians and mounted by experienced engineers. 
To the inadequate equipment of the amateur's workshop may 
therefore be ascribed many of the deficiencies in the mount- 
ings of reflecting telescopes. In some cases, however, re- 
flectors of large aperture, flgured and mounted by professional 
opticians and engineers, have given results of little or no 
value. In these cases, as in others, it appears that insufficient 
attention was paid to the excessive sensitiveness of large 
mirrors, which causes them to require much more careful 
treatment than is amply sufficient to yield good images with 
a lens. 

In stellar spectroscopic work good results were obtained 
with reflectors by Huggins and Draper at a comparatively 
early period, but it was not until the last years of the nine- 
teenth century that such telescopes were employed with any 
considerable degree of success for the photography of nebulae. 
The first photograph of a nebula was obtained with a refract- 
ing telescope by Draper in 1881. Photographs of the Great 
Nebula in Andromeda, made by Roberts in 1886 with a 
twenty-inch reflector, showed for the first time the truly 
spiral form of this remarkable object, and thus indicated 



42 Stellae Evolution 

some of the great possibilities of investigating nebular 
structure with instruments of this type. Briefly speaking, 
their superiority to refractors lies in the fact that the light 
is not weakened by passage through glass, but, after reflec- 
tion from a surface of pure silver, all the rays, independently 
of their color, are united in a common focus. With a 
refractor many of the rays are completely cut off during 
transmission through the glass of the lens, which is as 
impervious as so much steel to the very short waves of the 
ultra-violet spectrum. Furthermore, a lens does not unite 
all the rays of different colors into a single focus, but forms 
a series of images, corresponding to light of different wave- 
lengths. In order to get a sharp photograph with a refract- 
ing telescope, it is therefore necessary to discard some of 
these rays, in the manner already described (p. 35). The 
reflector, on the contrary, utilizes all of the light ^ — an advan- 
tage which is clearly shown by the results obtained with this 
type of telescope. 

The photographic studies of nebulae made by Keeler with 
the Crossley reflector of the Lick Observatory, mark a step 
of the greatest importance in the development of the reflecting 
telescope. The mounting of this instrument, constructed in 
England some years previously, and presented to the Lick 
Observatory by Mr. Crossley, was very poorly adapted to carry 
the excellent mirror of three feet aperture. But through the 
extraordinary efforts of Keeler, whose severe exertions in 
carrying out this work hastened his death, the mounting 
was so strengthened and improved as to permit remarkable 
results to be obtained. In other hands, even after these 
improvements had been made, it is doubtful whether such 
exquisite photographs would have resulted. But, after many 
unsuccessful efforts, Keeler learned how to overcome the 
difficulties peculiar to the instrument, and in this he has been 

1 Except a certain percentage lost in reflection. 



Development of Reflecting Telescope 43 

ably followed by Perrine.^ We shall have occasion later to 
refer to their results. 

The mounting of the two-foot reflecting telescope of the 
Yerkes Observatory was designed expressly for photographic 
purposes, and no pains were spared to adapt the instrument 
for the exacting requirements of such work. The mirror, 23J 
inches in diameter and of 93 inches focal length, was con- 
structed by Ritchey in 1895 at his home in Chicago. This 
mirror is of the highest quality, meeting the most severe 
optical tests that can be applied to it. The mounting of the 
telescope, designed by Wadsworth, with modifications by 
Ritchey, was constructed in the instrument shop of the Yerkes 
Observatory, and is very heavy and rigid. In the photograph 
(Plate XVIII) the mirror may be seen in position at the lower 
end of the skeleton tube. At the upper end of this tube is 
a small plane mirror, so supported that its face makes an 
angle of 45° with the axis of the tube. The telescope is 
therefore of the Newtonian type, the image being formed on 
the photographic plate near the upper end of the tube, after 
reflection of the cone of rays from the small mirror. The 
double slide plate-carrier, which holds a plate 3 J X 4J inches 
in size, is precisely similar to the plate-carrier employed with 
the forty-inch refractor (Plate XVII). 

A comparison of the results obtained with this instrument, 
with those secured with the forty-inch Yerkes refractor, will 
suffice to show the peculiar advantages of the reflector for 
certain kinds of work. It should not be forgotten that the 
forty-inch refractor has other advantages, which fit it for 
work that could not be done under any circumstances with 
the two-foot reflector.^ But in the photography of faint 

1 A new and satisfactory mounting has since been constructed for the Crossley 
reflector. 

^For example, the scale of the images given by the forty-inch is eight times that 
of the two-foot reflector. Moreover, the former is well adapted for work on the Sun, 
for which the latter cannot be used. 



44 Stellar Evolution 

stars, particularly in the photography of nebulae, the two- 
foot reflector is especially useful. It is possible with this 
instrument, in an exposure of only forty minutes, to photo- 
graph stars which are at the extreme limit of vision with the 
forty-inch refractor. With longer exposures, countless stars, 
which can never be seen or photographed with the large 
refractor, are recorded on the plates. Compare, for example, 
the photographs of the star cluster Messier 13, reproduced 
in Plates XIX and XX. The principal advantage of the 
reflector in such work, as already explained, is the con- 
centration of the light-rays, irrespective of their color, in 
a single focal image. 

The photographs of nebulae obtained by Ritchey with the 
two-foot reflector show in a remarkable way the beauty and 
delicacy of structure which characterize these objects. It 
will be seen from the illustrations in the plates that the nebulae 
are of many types, although the spiral form predominates. 
The Great Nebula in Orion (Plate XXI), which is the most 
brilliant of the larger nebulae, is of irregular form, and 
marked complexity of structure. Of very different pattern 
is the beautiful nebula in Cygnus, the delicate filamentous 
structure of which is admirably shown by Ritchey's photo- 
graph (Plate LXXXVII). In the nebulae which envelop the 
stars of the Pleiades (Plate LXXXVI) two very different 
types of structure are shown ; long parallel filaments predomi- 
nate, but there may also be seen in the photograph a mass of 
nebulosity resembling the flame of a torch blown by the wind. 
But although, as we shall see, evidences may be found of the 
relationship of the stars in these nebulae to the cloud-forms 
themselves, the spiral nebulae certainly appeal most strongly 
to the imagination. The largest of these, the Great Nebula in 
Andromeda^ is perhaps the most interesting object in the 
heavens (Frontispiece). Persistent attempts to measure the 
distance of this nebula from the Earth, made with the most 



Development of Reflecting Telescope 45 

powerful of modern instruments, have totally failed. We 
may therefore conclude that this distance is almost incon- 
ceivably great, and that therefore the dimensions of the 
nebula are so enormous as to be quite beyond comparison 
with those of the solar system. In the beautifully defined 
spiral character of this object, so clearly visible on the photo- 
graph, although beyond recognition in visual observations, 
we seem to see strong indications of motion with respect to 
the center. But hitherto, in spite of the careful comparison 
of photographs made many years apart, no evidence of such 
motion has been detected. This fact would tend to confirm 
what we already know from measurement, namely, that the 
nebula is exceedingly remote from the Earth, and that the 
phenomena which it exhibits are on a gigantic scale. We 
cannot doubt that the component parts are in motion, and 
that in the course of time evidences of this motion will come 
to light. But to detect them it is certain that the most 
powerful instrumental means will be required, and that long 
intervals of time must separate the photographs which are to 
be compared. 

The Great Nebula in Andromeda thus stands as the 
largest representative of that great class of nebulae which 
was first made known through Lord Bosse's discovery of the 
spiral nebula in the Hunting Dogs. From some of Bitchey's 
photographs we are fortunate in being able to illustrate other 
spiral nebulae, which differ in various particulars, but in 
all cases show clearly the spiral structure (Plates LXXXIX 
and XC). As already stated, Keeler's photographic investi- 
gations with the Crossley reflector have shown that while large 
objects of this kind are comparatively few, the sky is scattered 
over with an immense number of small ones. The investi- 
gation of these nebulae, with the great reflecting telescopes 
of the future, should lead to results of fundamental 
importance. 



CHAPTER VII 

ELEMENTARY PRINCIPLES OF SPECTRUM ANALYSIS 

The problem of determining the nature of the nebulae 
seemed to be placed beyond solution by telescopic means 
when it was found that star clusters exist in which the stars are 
so densely packed that they cannot be separately distinguished 
by any telescope. A photographic illustration of this is given 
in Plate XIX. In Plate XI we see a cluster easily resolved 
into its constituent stars. In the case of Messier 13, however, 
the photograph here reproduced might leave some doubt on 
the score of resolvability.^ Visual observations, better com- 
petent than photographic ones to settle this particular point, 
remove the doubt in the present instance. But other clusters 
are still more closely crowded, and it was easy to believe that 
the unresolved nebulae might be objects of this nature. The 
structure of such a nebula as that shown in Plate XC might 
also be supposed to favor such a view. Sir William Herschel, 
great not only as an observer, but as a philosopher who looked 
deep into the nature of things, was not deceived by these 
circumstances, and persisted in his belief that the nebulae 
are masses of uncondensed gas, differing essentially from 
clusters of stars. As evidence of the uncertainty which 
nevertheless existed, it must be added that Sir John Herschel, 
though himself a great philosopher, was led to a contrary 
conclusion. For him no nebula existed that could not be 
resolved with a sufficiently powerful telescope into a congeries 
of stars. Under these circumstances it is evident that some 
additional means of analysis must be called upon to solve the 
problem. For as telescopes increased in size the nebulae 
remained unresolved, showing that either they were in their 

1 Even the large-scale photograph in Plate XX does not separate the closest stars. 

46 



Principles of Spectrum Analysis 47 

nature unresolvable, or that far more powerful instruments 
would be required to reveal their constituent parts. 

This was the condition of affairs when Spencer boldly 
took issue with the astronomers. Convinced that the 
principle of evolution must operate universally, and that 
the stars must have their origin in the still unformed 
masses of the nebulae, he ventured to question the con- 
clusion that the resolution of nebulae into stars was only 
a matter of telescopic power. He had not long to wait for 
support, for at this juncture a new method of research, long 
previously foreshadowed by Fraunhofer's analysis of sunlight 
in the early part of the nineteenth century, suddenly pro- 
claimed its power of accomplishing many surprising results. 

It has been known since the time of Newton that when 
sunlight is passed through a prism, it is spread out into a 
band containing all the colors of the rainbow. In Newton's 
experiments the sunlight was admitted to the prism through 
a circular hole, and he consequently failed to see in the 
colored spectrum any of those breaks or dark lines that were 
found in later years to be so significant. Fraunhofer, on the 
contrary, examined sunlight which reached the prism from 
a narrow slit, placed at a considerable distance. He was 
rewarded by the discovery of a large number of dark lines, 
differing greatly from one another in intensity, and irregularly 
distributed through the spectrum. He measured the positions 
of these lines in the spectrum with care, and designated the 
more striking ones with the letters of the alphabet. His 
designations are still retained, and the dark lines of the solar 
spectrum are still called the Fraunhofer lines. But of the 
origin of these lines Fraunhofer had no knowledge. He 
found, indeed, that the lines seen in sunlight, while present 
in the light of the planets, were replaced by different lines 
in the spectra of some of the stars. But while he con- 
cluded that the cause of the lines did not reside in the 



48 Stellar Evolution 

Earth's atmosphere, he nevertheless failed to discover their 
true explanation, and thus did not perceive the possibili- 
ties of the science of spectrum analysis. 

Let us consider for a moment what happens when light 
is passed through a prism. We may assume the light to be 
derived from the glowing tilament of an incandescent lamp, 
placed just in front of a narrow slit. After passing through 
the slit a (Fig. 1) the divergent rays fall upon the lens h, 




FIG. 1 
Passage of Rays through a Prism 

which renders them parallel, and is known as the collimating 
lens. The parallel rays now meet the face of the prism c, 
through which they are transmitted. After passing through 
the prism the rays fall upon the lens d, precisely similar to 
the collimating lens, which forms an image on the screen e. 
Now, when light strikes a prism it is deviated from a 
straight path, and the amount of its deviation depends upon 
the color of the light. Yellow light, for example, is deflected 
by a prism more than red light. Green light is deflected 
more than yellow light, blue light suffers even a greater 
change of direction, while violet light is deflected most of all. 
It is thus evident that if the light from the incandescent 
lamp were pure red, and contained no other color, we should 
have a red image of the slit at R. If it were yellow, a yellow 
image of the slit would be formed at Y. Green light would 
form a green image of the slit at G, blue light a blue image 



Principles of Spectrum Analysis 49 

at J5, and violet light a violet image at T^. White light is 
compounded of all these colors, and shows every intermedi- 
ate gradation of tint. When passed through a prism it is 
therefore dispersed into a colored spectrum, extending from 
red at one end through yellow, green, and blue to violet. This 
is called a continuous spectrum, and is produced when the 
light from any white-hot solid body is analyzed by a prism. 
Liquids, or even gases when sufficiently compressed, may 
give a continuous spectrum when highly heated. But vapors 
and gases, under ordinary conditions, produce characteristic 
spectra of bright lines, by which they may be recognized. 

For example, let us replace the incandescent lamp flame 
by a non-luminous gas flame, such as is produced when gas 
is burned after being thoroughly mixed with air. If we 
introduce into this flame a little common salt, it will be 
instantly colored a deep yellow. This yellow light, after 
transmission through the slit and the prism, will produce 
upon the screen a single yellow line at the point Y. A more 
powerful instrument would resolve this line into two, placed 
very close together on the screen. But for our present pur- 
poses we may consider this to be a single line due to the 
metal sodium, which in conjunction with chlorine constitutes 
common salt. Wherever sodium is present in a state of vapor, 
whether in a flame, or between the carbon poles of an elec- 
tric arc, or in the atmosphere of the Sun, or in that of the 
most distant star, it gives rise to this line, which always lies 
at precisely the same point in the spectrum. With suffi- 
ciently powerful instruments the line is always double, and 
its presence, when accurately determined, is sufficient to 
prove the existence of sodium in any luminous source (Fig. 1, 
Plate XXII). 

Most substances, when their vapors are caused to radiate 
in this way, produce more than one colored image of the 
slit upon the screen. Thus strontium, when introduced into 



50 Stellar Evolution 

the flame, gives two red lines and a strong blue line. Potas- 
sium gives a line in the extreme red and another in the 
extreme violet. But the essential point to notice is that no 
two substances give lines at precisely the same place in the 
spectrum. From this we may conclude that the spectra are 
entirely characteristic of the various elements, and therefore 
that the presence of these elements in a state of vapor can 
always be recognized by the detection of their peculiar lines. 

The spectra of the elements are of all degrees of com- 
plexity, ranging from only two or three lines up to several 
thousand. Iron, for example, when turned into vapor in 
the electric arc, shows, after analysis by the prism, several 
thousand lines, irregularly distributed through all parts 
of the spectrum (a few of these are shown in Fig. 2, Plate 
XXII) . It is evident, therefore, if the lines are to be clearly 
distinguished from one another, and so accurately recognized 
as to avoid confusing a line of iron, for example, with one 
belonging to some other substance, that powerful dispersion 
may be necessary; i. e., the various lines must be separated 
from one another as far as possible by drawing out the spec- 
trum to a great length. This can be done by passing the 
light through several prisms in succession, rather than 
through a single prism, as in the present instance. 

So far we have referred to the spectra of metallic vapors, 
rendered luminous in the gas flame or in the electric arc. 
In order to obtain the characteristic spectrum of a gas, such 
as hydrogen, it may be placed in a tube, and made lumi- 
1 nous by an electric discharge. The best results are ob- 
tained after the pressure in the tube has been reduced by 
pumping out some of the gas, until the electric discharge 
passes quietly and continuously, so that the whole interior 
of the tube continues to glow with the light of its gaseous con- 
tents. This light, when analyzed by a spectroscope like that 
shown in Fig. 2, is found to give lines which are character- 



Principles of Spectrum Analysis 



51 



istic of the gas employed. The light of hydrogen in a vacuum 
tube, for example, gives precisely the same spectrum as the 
light of hydrogen proceeding from one of the great flames 
at the edge of the Sun. 

We have now considered two types of spectra: (1) the 
continuous spectrum, produced when a solid body, a liquid, or 




fig. 2 
Kirchhoff's Spectroscope 

a highly compressed gas, is rendered white-hot by sufficient 
heat; and (2) a hrighf-line spectrum, consisting of bright 
lines, irregularly distributed on a dark background, and de- 
rived from the prismatic analysis of the light emitted by 
luminous metallic vapors, or gases rendered incandescent 
by electric discharges. One other type of spectrum remains 
to be mentioned: a dark-line spectrum, such as Kirchhoff 
observed and explained when he effected his famous analysis 
of sunlight at Heidelberg in 1859. 

We have already remarked that Fraunhofer had noted 



52 Stellak Evolution 

the existence of dark lines in the continuous spectrum of the 
Sun, and accurately measured their positions, though with- 
out understanding their meaning. KirchhofP, using the four- 
prism spectroscope shown in Fig. 2, saw these same dark 
lines in the solar spectrum, and succeeded in explaining their 
origin. In the yellow part of the spectrum he observed two 
strong dark lines, very close together. When the sunlight 
was excluded from the spectroscope, and a gas flame contain- 
ing sodium vapor was placed in front of the slit, two strong 
bright lines, occupying exactly the same positions as the 
dark lines of the solar spectrum, were seen in their place. 
The flame was then copiously charged with sodium vapor 
and retained in its position in front of the slit, the sunlight 
being permitted to shine through it. It was immediately 
noticed that the two dark lines in the solar spectrum were 
considerably darker and more conspicuous when the sunlight 
passed through the sodium flame than when it was observed 
alone. Furthermore, it was found that when any white light, 
producing a continuous spectrum without lines, was allowed 
to shine through a flame containing sodium vapor, the effect 
of the flame was to produce two dark lines in the yellow, in 
the precise position of this conspicuous pair of dark lines. 

Iron, when transformed to luminous vapor in the electric 
arc, gave an even more convincing proof that the true expla 
nation of the solar spectrum had been found: the bright 
lines observed in its spectrum by Kirchhoff and Bunsen 
were seen to be represented in the solar spectrum by an 
equal number of dark lines, precisely resembling them both 
in position and in relative intensity. Magnesium, nickel, 
calcium, and other substances gave similar results, and the 
conclusion was irresistible that all of these substances exist 
in the Sun in a state of vapor. It followed from these exper- 
iments that the body of the Sun must be an intensely hot 
mass, emitting white light, which, if it could be observed 



Peinciples of Spectrum Analysis 53 

alone, would give a continnous spectrum, crossed by no lines 
of any kind. Surrounding this brilliant white sphere, the 
observations proved the existence of a cooler atmosphere con- 
taining, in a state of vapor, most of the metals known on the 
Earth. These vapors, though cooler than the central body 
of the Sun, are nevertheless intensely hot, their temperature 
undoubtedly exceeding that of the most powerful electric arc. 
Hence, if their light could be observed alone, they would be 
seen to give a very complex spectrum of bright lines, in which 
all of the lines characteristic of the different elements would 
be present. It will be shown later that such a spectrum of 
bright lines may be seen at the edge of the Sun, when the 
apparatus is so adjusted as to admit only the light of the 
chromosphere to the slit of the spectroscope, while excluding 
all of the light from the Sun's disk. The bright lines in 
this spectrum are less brilliant than the continuous spectrum 
due to the more highly heated body of the Sun. Hence, 
when observed against the disk, the bright lines, appear 
dark by comparison. The cooler metallic vapors were 
shown by Kirchhoff's experiments to be capable of ahsorhing 
the same rays which they themselves emit, and the feebler 
radiations, emitted by the vapors themselves, produce the 
dark lines of the solar spectrum.^ It is important to notice 
that these so-called dark lines are dark only by comparison, 
since it will be explained later that photographs of the Sun 
can be taken by the light of any of these lines with the spec- 
troheliograph, showing the distribution of the corresponding 
element in the solar atmosphere. 

It immediately became evident to students of astrophysics 
that the method of analysis initiated by Kirchhoff must prove 
immensely powerful in extending their researches. In 1862 
Huggins, Secchi, and Rutherfurd commenced their extensive 

1 In Fig. 1, Plate XXII, the two bright lines are due to very hot sodium vapor at 
the center of the arc. The cooler and less dense vapor in the outer arc produces, by 
absorption, the narrow dark lines seen superposed on the bright ones. 



54 Stellar Evolution 

observations on the spectra of stars, and soon established a 
system of types, based upon the examination of the spectra 
of several thousand objects. This work has since been greatly 
extended through the application of photographic methods, 
introduced by Huggins, and applied with marked success by 
Draper and many others. In 1868 the spectroscope was 
used for the first time to analyze the red flames seen during 
total eclipses of the Sun. Not only did it demonstrate their 
gaseous nature, but a short time later, through the efforts of 
Janssen, Lockyer, and Huggins, it was found possible to em- 
ploy the spectroscope to observe the forms of the prominences 
in full sunlight. 

These and other applications of the spectroscope will be 
more fully described in subsequent chapters. Our present 
purpose is to explain how the new method, in the hands of 
Huggins (Plate XXIII), finally proved beyond doubt that 
certain nebulae are to be sharply distinguished from star 
clusters. 

Sir William Huggins' account of his first spectroscopic 
examination of a nebula is recorded in the Publications of the 
Tulse Hill Observatory, Vol. I: 

On the evening of August 29, 1864, I directed the spectroscope 
for the first time to a planetary nebula in Draco. I looked into the 
spectroscope. No spectrum such as I had expected! A single 
bright line only! At first I suspected some displacement of the 
prism and that I was looking at a reflection of the illuminated slit 
from one of its faces. This thought was scarcely more than 
momentary; then the true interpretation flashed upon me. The 
light of the nebula was monochromatic, and so, unlike any other 
light I had as yet subjected to prismatic examination, could not be 
extended out to form a complete spectrum. After passing through 
the two prisms it remained concentrated into a single bright line, 
having a width corresponding to the width of the slit, and occupy- 
ing in the instrument a position at that part of the spectrum to 
which its light belongs in refrangibility. A little closer looking 
showed two other bright lines on the side toward the blue, all three 



Principles of Spectrum Analysis 55 

lines being separated by intervals relatively dark. The riddle of 
the nebulae was solved. The answer, which had come to us in the 
light itself, read: Not an aggregation of stars, but a luminous gas. 

With this advance a new era of progress began. The 
power of the spectroscope to distinguish between a glowing 
gas and a starlike mass of partially condensed vapors estab- 
lished it at once in the place it still holds as the chief instru- 
ment of the student of stellar evolution. It became apparent 
that the unformed nebulae might furnish the material from 
which stars are made. 

It must not be forgotten, however, that only a small 
number of nebulae give a spectrum of bright lines, showing 
them to be gaseous. Most of the nebulae, including the 
very numerous spiral type, have a continuous spectrum, in 
which no lines have yet been detected. As stars are almost 
certainly formed from these "white" nebulae, as well as 
from the "green" gaseous ones, the theory of stellar evolu- 
tion must be broad enough to embrace both types. 



CHAPTER VIII 

GRATING SPECTROSCOPES AND THE CHEMICAL 
COMPOSITION OF THE SUN 

The general process employed by KirchhofP to investi- 
gate the chemical constitution of the Sun has already 
been described, but it also seems desirable to give an 
account of the perfected method used for this purpose in 
a modern laboratory. In order to prove that a given sub- 
stance exists in the Sun, its lines must be identified with 
certainty in the solar spectrum. The spectrum of iron, for 
example, contains thousands of lines, and it might easily 
happen that through chance proximity many of these lines 
would appear to coincide with some of the exceedingly 
numerous lines of the solar spectrum. It is evident, there- 
fore, that the method of comparison adopted must be such 
as to permit of a high degree of precision in measuring the 
positions of the lines. In other words, the dispersion of the 
spectroscope must be so great as to give a very long spec- 
trum, in which the lines are well separated from one another. 
Thus their positions can be accurately determined, and there is 
no danger of confusion in the case of closely adjacent lines, 
which in a less powerful instrument might be seen as one. 

The recent great advances in spectroscopy have been due 
in very large measure to the success of Rowland in ruling 
gratings of high resolving power. In a previous chapter it 
was remarked that the dispersion of a spectroscope may be 
increased by increasing the number of prisms through which 
the light passes. This not only gives a longer spectrum; it 
also increases the resolving power of the instrument, or 
its capacity of separating closely adjacent lines. But 

56 



Grating Spectroscopes 57 

through the loss of light by reflection and absorption, which 
becomes very serious when many prisms are employed, 
a limit is soon set to the increase in resolving power of 
prism spectroscopes. It is for this reason that the grating 
has played so large a part in the recent development of 
the subject. For the resolving power of a perfect grating 
depends only upon the total number of lines it contains, and 
the light efficiency, per unit area, may be as great for a 
large grating as for a small one. 

The production of very powerful spectroscopes, through 
the use of large and accurately ruled gratings, is what Row- 
land succeeded in accomplishing in his epoch-making work 
at the Johns Hopkins University. An optical grating con- 
sists of a polished metallic surface, on which many equidis- 
tant lines are ruled with a diamond point. The perfection 
of the spectra given by such a grating depends upon the 
number of lines it contains and upon the accuracy of their 
spacing. The difficulty of Rowland's task will be appre- 
ciated when it is remembered that a grating must contain 
from 10,000 to 20,000 lines per inch, and that errors in the 
positions of the lines, amounting to a very small fraction of 
the interval between them, would affect the performance of 
the grating, tending to blur and confuse the spectra pro- 
duced by it. 

Gratings that gave very good results were made many 
years ago by Rutherfurd, of New York, but it remained for 
Rowland to surpass them, both in quality and in size. His 
celebrated ruling-engines (Plate XXIV), which are still in 
regular use in the underground constant-temperature vaults 
of the physical laboratory at the Johns Hopkins University, 
depend for their success upon the fact that the screw, which 
is employed to move the grating-plate forward by about 
1/15,000 of an inch between successive strokes of the dia- 
mond, contains almost no errors. It cannot be said, of course, 



58 Stellar Evolution 

that the screw is entirely free from error, but the effect of 
the exceedingly minute irregularities is almost wholly com- 
pensated by ingenious devices that form a part of the rul- 
ing-engine. The machine is automatic in its action, and 
when set in motion the ruling of a large grating goes on 
without interruption for six days and nights before it is 
completed. 

The gratings manufactured on Rowland's machine have 
gone into observatories and laboratories in all parts of the 
world, where they have been the principal agents of spectro- 
scopic research during the last quarter of a century. Their 
great efficiency has caused them to displace prisms from 
nearly all spectroscopes in which very high resolving power 
is required. As we shall see later, however, the prism still 
remains of great importance to the spectroscopist, particularly 
in work requiring moderate resolving power, where it gives 
a much brighter spectrum than a grating. 

Rowland's contributions to spectroscopy were by no means 
confined to the manufacture and distribution of optical grat- 
ings. In addition to his very extensive researches on the 
solar spectrum, and on the spectra of the elements, he 
invented the concave grating, which now forms an essential 
part of the powerful spectroscopes found in many labora- 
tories. Prior to Rowland's time the comparatively few 
gratings which had been made were ruled on plane surfaces, 
and employed with the ordinary collimator and telescope 
of the laboratory spectroscope. That is to say, the prism of 
an ordinary spectroscope was removed, and the grating sub- 
stituted for it. In such an instrument the rays of light, after 
passing through the slit, fall upon the collimator lens, which 
renders them parallel. The parallel rays then meet the sur- 
face of the grating, where they are diffracted and spread out 
into a spectrum. This spectrum is observed or photographed 
with the aid of a second lens, which forms its image on the 



Grating Spectroscopes 



59 




retina or on a sensitive plate. A large spectroscope of this 
kind, used with the 40-inch Yerkes telescope for spectroscopic 
observations of the Sun, is illustrated in Plate XXX. 

Rowland showed, from theoretical considerations, that, 
if the grating were given a concave spherical surface, the 
collimator lens, and the observ- 
ing telescope as well, might be 
entirely dispensed with. He 
also devised the form of mount- 
ing for a concave grating illus- 
trated in Fig. 3. In the diagram, 
a is the slit through which the 
light enters, h the concave grat- 
ing, and c the eye or photo- 
graphic plate. It will be seen 
that no lenses enter into the 
construction of the apparatus; 
for some classes of work this is 
a point of great advantage. In 

the largest gratings used by Rowland the radius of curvature 
of the grating-plate, which is equal to the distance between 
the grating 6 and the photographic plate c, is 21 feet. The 
spectrum given by such a grating is many feet in length, 
and a portion of the spectrum 20 inches long or longer can 
be recorded by a single exposure on the photographic plate. 
In order to pass from one part of the spectrum to another, 
the grating-carriage h is moved along the rail ab, which 
causes the plate-carriage c to move toward or away from the 
slit on the rail ac. The whole apparatus is set up on piers 
in a dark room, to which no light is admitted except that 
which passes through the slit of the spectroscope. 

It should be remarked that a grating, unlike a prism, 
produces not merely a single spectrum, but several spectra, 
which can be observed successively by moving the carriage c 



FIG. 3 

Diagram of a Concave Grating 
Mounting 



60 Stellar Evolution 

along the track away from the slit. The first-order spec- 
trum lies nearest the slit. The second-order spectrum, twice 
as long as the first, which it partially overlaps, lies farther 
from the slit. The third and fourth orders, of increasingly 
higher dispersion, lie still farther from the slit. Only a por- 
tion of the fifth order can be observed with this instrument, 
and the higher orders, also beyond reach, are usually too 
faint to be of any service. 

Let us suppose that we wish to determine with such a 
spectroscope whether iron exists in the Sun. To accomplish 
this, sunlight must be reflected from the mirror of a heliostat 
(driven by clock-work, to maintain the beam in a fixed direc- 
tion) to the slit. Between the slit and the heliostat a lens is 
introduced, for the purpose of forming an image of the Sun 
upon the slit. When the illumination is secured in this way, 
the whole grating is filled with light from the diverging rays. 
The grating then produces an image of the solar spectrum 
upon the photographic plate, where it may be recorded by 
giving a suitable exposure. 

To facilitate an accurate comparison, the solar spectrum 
is photographed side by side on the same plate with the 
spectrum of the substance whose presence in the Sun is to 
be determined. In order to accomplish this, one-half of the 
slit is covered, and the sunlight is admitted through the 
other half. Thus the solar spectrum is photographed on 
one side of the plate. After this exposure is completed, the 
sunlight is shut off, and the screen in front of the slit moved 
so as to cover the half previously open, and to uncover the 
other half. The image of the Sun on the slit of the spectro- 
scope is then replaced by an image of an electric arc light, 
burning between two poles of iron. The spectrum of the iron 
vapor is thus produced on the plate, and a strip of this 
spectrum is photographed beside the strip of solar spectrum. 
This is illustrated in Fig. 2, Plate XXII, where the upper 



Grating Spectroscopes 61 

strip is a small part of the spectrum of iron. It will be seen 
by a glance at this photograph that these bright lines of iron 
are represented in the solar spectrum by corresponding dark 
lines, which accurately match them in position. In Rowland's 
work on the solar spectrum thousands of lines were found to 
correspond with iron lines given by the electric arc. 

The same process can be employed to determine the pres- 
ence of other substances in the Sun. In the case of metals, 
the electric discharge may be caused to pass between two 
metallic rods, or fragments of the metal may be placed in a 
hole drilled in one of the carbons of an ordinary electric arc- 
lamp. In the latter case the spectrum of carbon, and also of 
the impurities which the carbon poles always contain, will 
appear on the plate with the spectrum of the metal in ques- 
tion. But these extra lines may always be identified, and 
usually give no trouble. The identification of the solar 
lines, however, is not always so simple as in the case of iron. 
Many substances are doubtfully represented in the Sun by 
only a small number of lines, and it is sometimes very 
difficult to decide whether a few apparent coincidences are 
sufficient to warrant one in drawing definite conclusions. 
The matter is usually determined by ascertaining whether 
certain well-known groups of lines, which for various reasons 
are considered to be especially characteristic of an element, 
are actually represented. If these groups are absent, an 
apparent coincidence with certain less characteristic lines 
belonging to the same element should be regarded with 
suspicion. In the case of gases, the comparison is effected 
by the aid of vacuum tubes, in which the gas, usually at low 
pressure, is illuminated by an electric discharge. Thus the 
lines given by a hydrogen tube in the laboratory have been 
shown to coincide in position with lines ascribed to hydrogen 
in the Sun. 

After many years of study of the solar spectrum by these 



62 Stellar Evolution 

methods, Rowland reached the conclusion that the chemical 
composition of the Sun closely resembles that of the Earth. 
Certain elements, such as gold and radium, iodine, sulphur, 
and phosphorus, chlorine and nitrogen, have not been 
detected in the Sun. But this does not prove that they are 
certainly absent, as their level in the solar atmosphere, or 
the low degree of their absorptive effects might prevent 
them from being represented. On the other hand, various 
substances, not yet found on the Earth, are shown by many 
unidentified lines of the solar spectrum to be present in the 
Sun. Some, if not all, of these, will probably be discovered 
by chemists, just as helium was found by Ramsay in cleveite 
(p. 78). Rowland remarked that if the Earth were heated 
to a sufficiently high temperature, it would give a spectrum 
closely resembling that of the Sun. 

The most perfect maps of the solar spectrum are those of 
Rowland and Higgs. These are enlarged from photographs 
made with the concave grating, and contain an immense 
number of lines. Both maps extend into the extreme ultra- 
violet spectrum (the invisible region beyond the violet), and 
that of Higgs includes a considerable region of the infra-red 
(also invisible to the eye) where photographic plates sensi- 
tized for red light with alizarin blue or other dyes must be 
employed. Both maps are provided with scales of wave- 
length, so that the approximate positions of the lines can 
be read off at once. The precise positions of all solar lines 
photographed by Rowland are given in his Preliminary 
Table of Solar Spectrum Wave- Lengths^ which records the 
places of about 20,000 lines. This table, although known 
to contain some small errors, is at present employed by all 
spectroscopists as the standard of reference. It gives Row- 
land's identifications of the solar lines, but about two- 
thirds of the lines have not yet been referred to any known 
element. Recent investigations of the spectra of various 



Grating Spectroscopes 63 

metals will no doubt permit a considerable number of these 
lines to be identified. 

In any examination of the solar spectrum the observer 
cannot fail to be struck by the changing appearance of the 
lines in certain regions. In the yellow part of the spectrum, 
for example, near the well-known D lines of sodium, the 
most casual examination will show surprising variations in 
the intensity of the countless lines which are frequently 
conspicuous here. How great the change is may be seen 
in Plate XXV, which is a reproduction of two photographs 
of this part of the spectrum taken under different conditions. 
The lines which thus change in intensity are called telluric 
lines, since they are due to the absorption of the gases in 
the Earth's atmosphere. The region illustrated in Plate 
XXV contains a large number of lines due to water vapor. 
Since the amount of water vapor undergoes great variations, 
it is natural that the intensities of the lines should change 
accordingly. 

All of the telluric lines are most conspicuous in the 
spectrum of the Sun when it is near the horizon, since in this 
case the light traverses a very great depth of atmosphere 
before it reaches the spectroscope. Photographs of the 
spectrum of the high and low Sun might therefore be 
expected to show marked differences in the intensity of the 
telluric lines. This is actually the case, and the method 
therefore affords one means of identifying lines due to the 
absorption of our atmosphere. The oxygen in the air pro- 
duces two similar groups (A and B in Fraunhofer's original 
designation of the solar lines) which lie at the extreme red 
end of the solar spectrum. Cornu observed these same 
groups in the spectrum of an electric light at the summit of 
the Eiffel Tower, as seen from the Ecole Polytechnique in 
Paris, at a distance of about 2.7 miles. 

An ingenious method was employed by Cornu to distin- 



64 Stellar Evolution 

guish the telluric lines from those due to absorption in the 
Sun's atmosphere. According to Doppler's principle, the 
lines in the spectrum of the east limb of the Sun must be 
displaced toward the violet (by motion of approach), and 
those from the west limb toward the red (recession), since the 
Sun is rotating on its axis in a period of about twenty-five 
days. It occurred to Cornu that this fact might give a very 
delicate means of picking out the telluric lines, since only the 
lines of solar origin can be displaced by the Sun's rotation, 
while those due to absorption in the Earth's atmosphere 
remain in their normal positions. He formed a small image 
of the Sun on the slit of his spectroscope, by means of a lens 
which could be made to oscillate rapidly, thus causing the east 
limb and the west limb of the Sun's image to fall alternately 
upon the slit. If the spectrum is observed while the image 
is oscillating, the lines of solar origin will be seen to move 
rapidly to and fro through a short distance, while the telluric 
lines will remain fixed. This method was successfully em- 
ployed by Cornu in an important study of the telluric lines. 
Other investigations of these lines, which have resulted in 
the production of extensive maps, have been made by ThoUon 
(continued by Spee), Becker, and McClean. In these inves- 
tigations the telluric lines were distinguished by observations 
of the spectrum of the high and low Sun. 

If passage of sunlight through our atmosphere is thus 
competent to produce dark lines in the solar spectrum, it is 
evident that the sunlight reflected from a planet should show 
evidence of its double transmission through the planet's 
atmosphere. This method is actually employed to determine 
the presence and the composition of the atmospheres of the 
planets. 

Remarkable as was Rowland's success in the manufacture 
of gratings, and the measurement of wave-lengths with their 
aid, it has recently been surpassed by Michelson. With the 



Grating Spectroscopes 65 

interferometer, an instrument of his invention, Michelson 
has established the length of the standard meter of the Inter- 
national Bureau of Weights and Measures, in terms of light- 
waves. This fixes, with the greatest precision, the wave- 
length of certain lines in the spectrum of cadmium, and these 
wave-lengths were adopted at the Oxford meeting ( 1905) of 
the International Union for Co-operation in Solar Research 
as primary standards, on which a new system of wave- 
lengths, to replace Rowland's system, will be based. Through 
his invention of the echelon, Michelson has realized a new 
form of grating, composed of a series of glass plates, precisely 
equal in thickness, piled one on another like a flight of steps, 
through which a parallel beam of light is transmitted. The 
spectra thus produced are of a very high order, and the 
resolving power surpasses that of Rowland's largest gratings. 
The echelon spectroscope thus furnishes the means of analyz- 
ing compound lines, the members of which lie so close to- 
gether that they cannot be separated with other instruments. 
The latest success achieved by Michelson, however, opens 
up still greater possibilities in spectroscopy. The echelon 
can be used only for the study of narrow and sharply defined 
lines; its application is therefore limited to certain special 
problems. For more general work, both in the laboratory 
and in the solar observatory, very large gratings, of high 
resolving power, are required. Six-inch gratings (ruled on 
a disk of speculum metal 6 inches in diameter) were success- 
fully made by Rowland. After several years of labor 
Michelson has completed a ruling-machine with an almost 
perfect screw, on which he has already made 8-inch and 10- 
inch gratings. He hopes to produce a 14-inch grating, the 
largest for which his machine is designed. There is reason 
to believe that his plan for constructing a ruling-machine 
with four screws, which should reduce the error to one-fourth 
its amount in a single screw machine, would result in the 



66 Stellae Evolution 

production of good 20-inch gratings. The enormous impor- 
tance of such gratings, in their application to the study of the 
Sun, will become clearer as we proceed. One difficulty to 
be overcome will be recognized when it is remembered that 
a 20-inch grating, having 12,500 lines to the inch, would 
contain more than 2,000,000 lines, each about 10 inches long. 
The microscopic diamond crystal, used to cut all these lines 
in the hard surface of the speculum metal, must not break, or 
change its form appreciably, during the entire period of the 
work. 

It is satisfactory to add that Jewell has recently con- 
structed a new ruling machine at the Johns Hopkins Univer- 
sity which appears likely, from preliminary tests, to be supe- 
rior to Rowland's. We thus have good reason to hope that 
the best existing photographs of the solar spectrum will soon 
be surpassed. 



CHAPTER IX 
PHENOMENA OF THE SUN^S SURFACE 

The results described in the last chapter relate to the 
light of the Sun as a whole, and not to the details of its sur- 
face phenomena. In most of the investigations there de- 
scribed similar results might be attained if the Sun were 
removed to the distance of the nearer stars. In that case it 
would no longer be possible, even with the most powerful 
telescope, to detect an appreciable disk, and the solar image 
would be reduced to a microscopic point, brilliant enough, 
however, to afford sufficient light for spectroscopic examina- 
tion. But it has already been pointed out that investigations 
of the Sun acquire their greatest importance through the 
comparative proximity of this star to the Earth. All other 
stars are so far away that no distinction can be drawn be- 
tween the radiations characteristic of different parts of their 
disks. The spectroscopist must therefore be content to 
observe in such cases a composite spectrum, produced by the 
superposition of the spectra of the various surface phenom- 
ena. The Sun, on the other hand, is so near us that its 
image at the focus of a powerful telescope may have a diam- 
eter as great as 7 inches, or even greater.^ Consequently, 
the light from any point in this image, corresponding to a 
small area of the solar surface, can be studied by itself. Our 
extensive knowledge of the Sun, except that which has been 
derived from an examination of its light as a whole, is based 
upon this fact. 

The appearance of the Sun in a telescope is illustrated by 
Plate II, which is a reproduction of a direct photograph. 

1 The actual diameter of the Sun is about 860,000 miles. 

67 



68 Stellar Evolution 

The Sun's light is too brilliant to permit of visual observa- 
tion without some method of reducing its intensity. The best 
means of accomplishing this is by the aid of the polarizing 
helioscope, which is attached just in front of the eye- 
piece of the telescope. The cone of light from the object- 
glass meets a plane surface in the helioscope, from which it 
is reflected at an angle such as to polarize the rays. As is 
well known, the amount of plane polarized light which can 
be reflected from a second surface depends upon the angle 
at which the rays meet this surface. Consequently, by 
rotating the reflecting prism the amount of light which 
reaches the eye can be varied at will, thus producing an 
image of any desired brightness. When protected by this 
device, the eye of the observer of solar phenomena is sub- 
jected to even less strain than is frequently experienced in 
work on fainter objects. 

A casual glance at the solar image is sufficient to show 
that it is much darker near the circumference than at the cen- 
tral part of the disk. This falling-off in brightness toward 
the limb is probably due to the absorption of a smoke-like 
envelope, w^hich completely incloses the Sun. The absorp- 
tion is so marked that near the circumference of the Sun only 
about 13 per cent, of the violet rays escape. For the blue, 
green, and yellow rays the percentage of transmitted light 
increases progressively, until it amounts to about 30 per cent, 
for the red. It has therefore been concluded that, if this 
absorbing atmosphere were removed, the color of the Sun 
would appear blue, since the intensity of the violet rays 
would be about two and one-half times as great as at present, 
while the red rays would be only about half again as bright 
as they are now. 

The visible phenomena of the Sun's disk include the sun- 
spots and the faculae. The general appearance of sun-spots, 
when seen with a low magnifying power, is shown in Plate II. 



Phenomena of the Sun's Surface 69 

Under perfect atmospheric conditions, a large sun-spot, when 
observed with a powerful telescope, would more closely 
resemble Plate XXVI, which is reproduced from a drawing 
made by Langley. The best solar observers agree that this 
drawing is one of the most perfect representations of spot 
structure yet obtained. The long narrow filaments, which 
constitute the penumbra of the spot, reach in toward a dark 
central region, called the umbra. It must be remembered 
that the darkness of the umbra is only relative: if observed 
alone, and not in contrast with the more brilliant surround- 
ings of the photosphere, the great brilliancy of the umbra, 
surpassing that of the most powerful electric arc-light, would 
be evident. Knowledge of this fact has been quite suf- 
ficient to set at rest the old notion that sun-spots are 
merely rents in a brilliant cloud-covering of the Sun, through 
which a dark and cool interior may be seen. 

According to Langley's view, the filaments which, taken 
together, constitute the penumbra are everywhere present 
on the solar surface. He regarded them as resembling the 
stalks of a wheat-field, seen on end in the undisturbed pho- 
tosphere, and revealing more of their true characteristics in 
the penumbra, where they are bent over and drawn out 
toward the central part of the spot. Langley believed that 
we are observing columns of luminous vapors rising from the 
Sun's interior, the seats of convection currents which bring 
to the surface the immense supplies of heat radiated by the 
Sun into space. Separating these luminous columns are 
darker regions, characterized by a lower degree of radiation. 

Such minute details can be recorded only with the greatest 
difiiculty. Under ordinary atmospheric conditions the solar 
image is not seen as a sharp and well-defined object, but its 
details are continually blurred by the effect of irregularly 
heated currents in our atmosphere. Even under the best 
conditions the moments of very sharp definition are few, and 



70 Stellar Evolution 

the greatest patience and perseverance are required on the 
part of an observer who wonld record his impressions of the 
solar structure. At the best, drawings based upon visual 
observations must be unsatisfactory, since even the skilled 
hands of Langley could not secure the perfect precision 
which is so desirable. It accordingly might be hoped that 
here, as in other departments of solar research, photography 
would afford the necessary means of securing results unat- 
tainable by the eye. Unfortunately, however, this hope has 
been only partially realized, as a brief consideration of the 
best results in this field will show. 

It is a comparatively simple matter to make a direct 
photograph of the Sun. It is only necessary to form a solar 
image, considerably enlarged, upon a "slow" photographic 
plate, and then to give an excessively short exposure by 
means of a shutter containing a narrow slit, which is shot 
across just in front of the plate at very high speed. The light 
from any part of the Sun reaches the plate only during the 
brief interval in which the slit is passing the corresponding 
part of the image. The exposure for any point may thus 
amount to no more than rTroVoir ^^ ^ second. The photo- 
graph reproduced in Plate II was taken in this way. 

In order to obtain photographs showing the smaller 
details of the photosphere, it is desirable to use a solar 
image enlarged to a diameter of from 15 to 30 inches, with 
photographic plates particularly adapted for this class of work. 
The best direct photographs hitherto made are those taken 
by Janssen at the Observatory of Meudon, near Paris. A 
portion of one of these pictures, representing the great Sun- 
spot of June 22, 1885, is reproduced in Plate XXVII. The 
penumbra is not very well shown, since the exposure required 
for the brighter regions of the surrounding photosphere is 
too short to bring out its fainter details. Even with sufficient 
exposure, however, such photographs do not reveal the more 



Phenomena of the Sun's Surface 71 

delicate details recorded in Langley's drawings. But they 
do show, with considerable success, the minute structure of 
the photosphere, as Plate XXVII illustrates. Here may be 
seen, autographically recorded, the photospheric "grains" 
which Langley believed to be the extremities of long fila- 
ments reaching down toward the interior of the Sun. Jans- 
sen holds a different view, since he regards the bright grains 
to be small spherical masses of luminous vapor, separated by 
vacant regions. In chap, xi it will be shown that the results 
of recent investigations with the spectroheliograph tend to 
bear out Langley's view. 

There can be little doubt that direct photographs of the 
Sun, showing smaller details than have yet been registered, 
will ultimately be obtained. Janssen's photographs have all 
been secured with a small instrument, used in an atmosphere 
where the conditions are not particularly favorable for work 
of this character. It is therefore to be hoped that, with much 
more powerful apparatus, employed in a better atmosphere, 
the results would be still more satisfactory. 

In spite of long study and much discussion, it remains 
uncertain whether sun-spots are to be regarded as cavities or 
as elevated regions of the photosphere. At one time it was 
supposed, mainly as the result of observations made by 
Wilson, of Glasgow, in the eighteenth century, that sun-spots 
were saucer-shaped cavities, the penumbra representing the 
sloping edge of the saucer, with the umbra at the center. 
More recent investigations, however, have failed to confirm 
Wilson's observations, though there can be little doubt that 
the umbra lies below the level of the faculae that usually sur- 
round spots. Faculae are elevated regions of the photo- 
sphere, and the question remains open whether the level of the 
umbra is above or below the average level of the photosphere, 
outside of the faculae. 

The only other phenomena visible in direct observations 



72 Stellae Evolution 

of the Sun are the faculae. They are usually most numerous 
in the vicinity of Sun-spots, and near the Sun's limb they 
are sometimes very conspicuous brilliant objects, covering 
large areas. Near the center of the Sun, however, they 
are practically invisible, though faint traces of them can 
sometimes be made out on photographs taken with a suitable 
exposure. This increase of brightness toward the Sun's limb 
is assumed to be due to the elevation of the faculae above the 
photospheric level, and their escape from a considerable part 
of that absorption which so materially reduces the brightness 
of the photosphere. Rising above the denser part of the 
absorbing veil, and thus suffering but little diminution of 
light, they appear near the Sun's limb as bright objects 
on a less luminous background. 

Janssen's photographs tend to bear out the assumption 
that the faculae resemble the rest of the photosphere, differ- 
ing mainly in their greater altitude. They are shown by 
these photographs to be resolved into granular elements 
similar to those that constitute the photosphere. It will be 
seen later, however, that the faculae play a very important 
role, since they are the regions from which immense masses of 
vapors rise to the solar surface. These vapors are invisible 
to the eye, and no trace of them is shown on photographs 
taken in the manner described above. But they may be 
photographed with the spectroheliograph, by the method 
explained in chap. xi. 



CHAPTER X 

THE SUN'S SURROUNDINGS 

The first observations of the Sun's surronndings date back 
to an early period. On the occasion of a total eclipse the 
dark body of the Moon covers the solar disk, cuts off the 
sunlight which at other times illuminates our atmosphere, 
and reveals phenomena ordinarily hidden by its glare. It is 
well known that, if our atmosphere were absent, there would 
be no such scattering of the sunlight, and the sky would be 
as dark during the day as it now appears at night. In such 
a case the stars would be visible by day, as well as the solar 
corona. Formerly, when no artificial means of reducing this 
brilliant illumination of the atmosphere were known, all 
knowledge of celestial phenomena in the immediate vicinity 
of the Sun was of necessity obtained during total eclipses. 
The solar corona was thus discovered, and likewise the red 
flames, or prominences, which do not extend so far from the 
Sun's surface. 

The corona still remains a mysterious phenomenon, since 
no means has yet been discovered of observing it without an 
eclipse. Our knowledge is thus confined to the results of 
observations made during the very brief periods when the 
Moon shields our atmosphere from illumination by the sun- 
light. The general appearance of the corona, as seen at 
the eclipse of May 28, 1900, is illustrated in Plate IV, 
reproduced from a photograph made by Barnard and 
Ritchey. It may be described as a faintly luminous veil 
of light, extending outward in long streamers from the sur- 
face of the photosphere to distances of several millions of 
miles, and exceeded in brilliancy, even in its brightest 

73 



74 Stellar Evolution 

parts, by the full Moon. In many ways its streamers re- 
semble those of the aurora borealis, and it is indeed possible 
that their origin may be ascribed to some similar electrical 
cause. During the few minutes of a total eclipse they are 
not seen to undergo change of form, but the outline of the 
corona does vary greatly from year to year, in sympathy 
with the general variation of the solar activity described in 
another chapter. At times of minimum sun-spots the form of 
the corona resembles that shown in Plate IV. This minimum 
type is marked by great winglike extensions along the solar 
equator, and by much shorter streamers, diverging like fans 
near the Sun's pole. At times of maximum sun-spots the 
corona is much more extensive in the polar regions, the 
streamers equaling in length those of the equatorial zone. 

Spectroscopic observations have shown that the corona con- 
sists mainly of gases unknown to the chemist. That is to 
say, the lines in its spectrum do not coincide in position with 
the lines of any terrestrial element. Whether these gases, 
which are probably very light, will ultimately be found on 
the Earth cannot be predicted. Like helium, first known in 
the Sun, they may eventually be encountered, in minute 
quantities, in some mineral, where they have hitherto escaped 
the chemist's analysis. The fact that the lower part of the 
corona gives a continuous spectrum, with a feeble solar spec- 
trum superposed upon it, indicates that minute incandescent 
particles are present, which are hot enough to radiate white 
light, and which scatter enough sunlight to account for the 
presence of the solar spectrum. 

It may now be of interest to explain how the solar corona 
is photographed during a total eclipse of the Sun, especially 
as the same means are employed during eclipses in photo- 
graphing the solar prominences, and also because we shall 
have occasion in a subsequent chapter to refer more at 
length to the general type of telescope here represented. 



The Sun's Surkoundings 75 

It has already been explained that the size of an image of 
the Sun given by a telescope depends directly upon the focal 
length of the lens employed. In order to show as distinctly 
as possible the more minute phenomena of the corona, it is 
therefore desirable to obtain large-scale photographs of it 
with a telescope of great focal length. Obviously such an 
instrument as the 40-inch Yerkes refractor could not easily 
be transported to the more or less remote regions of the 
Earth where the passing shadow of the Moon may render a 
total eclipse visible. Fortunately, however, the size of the 
focal image does not depend upon the diameter of a lens, but 
merely upon its focal length. Hence the desired result can 
be obtained by using a long-focus lens of comparatively 
small diameter. In some cases such lenses are pointed 
directly at the Sun, and the motion of the solar image, 
caused by the Earth's rotation, is compensated for by a cor- 
responding motion of the photographic plate on which the 
image falls. Another method, which offers various points of 
advantage, is illustrated in Plate XXVIII, reproduced from 
a photograph of the horizontal telescope used by the eclipse 
party of the Yerkes Observatory at Wadesboro, North Caro- 
lina, on May 28, 1900. 

The essential feature of this instrument is a plane mirror, 
12 inches in diameter, which reflects the Sun's rays hori- 
zontally through a long tube. The plane of the mirror is 
parallel to the Earth's axis, and, by means of an accurate 
driving-clock, the mirror is made to complete a rotation once 
in 48 hours. Such a motion of the mirror is just suflScient to 
counteract the effect of the Earth's rotation, and thus to keep 
the Sun's rays reflected in the same direction for an indefinite 
period. After leaving the coelostat mirror, the rays fail upon 
a 6-inch photographic lens, which forms an image upon a 
sensitive plate at its focus, 61^ feet away. Through the 
rotation of the mirror the image is maintained at a fixed 



76 Stellak Evolution 

position upon the photographic plate, so that any desired 
exposure may be given. 

With this apparatus some remarkably fine photographs of 
the corona and prominences were obtained by Barnard and 
Ritchey, of the Yerkes Observatory party (Plate XXIX). 
During the 87 seconds of the eclipse seven exposures were 
made, ranging in length from ^ second to 30 seconds. Several 
of the photographic plates used were 25 X 30 inches in size. 
To facilitate rapid handling, they were mounted on a wooden 
carrier 15 feet long, free to move on ball bearings on steel 
rails extending at right angles to the tube through the en- 
tire length of the photographic house. A catch, operated by 
hand, served to stop the plate-carrier at the proper position 
for each exposure. 

The solar prominences are seen at total eclipses of the 
Sun, projecting like red flames beyond the dark edge of the 
Moon (Plate XXIX). With our present knowledge of these 
phenomena, it seems hardly possible that just prior to the 
middle of the last century they were regarded by some 
observers as the illuminated summits of lunar mountains. 
Their truly solar origin was conclusively demonstrated in 
1860, when they were photographed by Secchi and de la 
Rue, and were shown not to share the motion of the Moon. 
At that time, however, no conclusions could be drawn as to 
their chemical composition, and it was not until 1868 that 
their gaseous nature and their connection with the Sun 
became known through the use of the spectroscope. It 
was then found that these immense masses of hydrogen and 
helium gas rise from a sea of flame (the chromosphere, which 
completely envelops the Sun), and sometimes attain eleva- 
tions of hundreds of thousands of miles. 

The rarity and brief duration of total eclipses would have 
limited greatly our knowledge of the prominences, had not 
Janssen, Lockyer, and Huggins devised an epoch-making 



The Sun's Surroundings 



77 




method by which they can be observed on any clear day, in 
spite of the glare of our atmosphere near the Sun. The 
instrument which permits this result to be accomplished is 
the spectroscope, used 
in conjunction with a 
telescope. The prin- 
ciple of the method is 
simple and easily un- 
derstood. The white 
light of the sky, when 
passed through the 
spectroscope, is drawn 
out into a long rain- 
bow band, and there- 
by greatly reduced in 
intensity. The light 
of the prominences, 
on the contrary, is 
concentrated in the radiations characteristic of hydrogen and 
helium gas, and the dispersing power of the spectroscope 
merely separates more and more widely the colored images 
which correspond to these radiations, without seriously en- 
feebling them. With the spectroscope they therefore become 
visible, since their images are brighter than the highly dis- 
persed background of skylight on which they lie. 

Plate XXX shows a solar spectroscope suitable for ob- 
serving the spectra or the forms of the prominences in full 
sunlight.^ This spectroscope consists essentially of a slit 
(s in the accompanying diagram, Fig. 4), which may be set 
tangentially or radially upon the Sun's limb; a collimating 
lens, Z, which renders parallel the rays coming to it from 
the slit; a plane grating, g, ruled with about 15,000 lines to 



FIG. 4 
Diagram of Solar Spectroscope 



1 This spectroscope, here shown attached to the Yerkes telescope, was formerly 
used as a spectroheliograph with the Kenwood telescope (Plate XXXIV;. 



78 Stellar Evolution 

the inch; and a second lens and eye-piece, / and e, which 
form the observing telescopb. The grating diffracts the light 
which reaches it from the \collimating lens, and produces a 
spectrum, an image of which is formed by the lens t, in the 
focal plane of the eye-piece e. If it is desired to photograph 
the spectrum, the eye-piece may be replaced by a sensitive 
plate. 

If we wish, for example, to observe the spectrum of the 
chromosphere with this instrument, the slit, about 1/1,000 
of an inch wide, is made exactly tangential to the solar image. 
Under these circumstances the observer at the instrument 
will see the spectrum of the bright sky near the Sun, which 
is of course merely the spectrum of reflected sunlight, and is 
therefore crossed by all of the dark Fraunhofer lines. In the 
case of substances which are present in the chromosphere, the 
lines of the spectrum will be reversed from dark to bright in 
regions which correspond to the section of the chromosphere 
lying upon the slit. The most conspicuous bright lines to be 
observed in this way are the hydrogen lines Ha (red), H^ 
(blue-green), H'y (blue), and H^ (violet), and the brilliant 
yellow helium line Dg. 

The history of this helium line affords an interesting 
illustration of the intimacy of the relationship which now 
unites terrestrial and solar chemistry. In his first observa- 
tions of the spectrum of the prominences, made in 1868, 
Lockyer was attracted by the presence of a bright line in the 
yellow, not far from the position of the Dj and Dg lines of 
sodium. This line was designated as Dg, but all attempts to 
identify it among the lines of known elements were unsuc- 
cessful. Accordingly, it was assumed to represent a new gas, 
probably very light, on account of its association with hydro- 
gen at great elevations above the solar surface. Lockyer 
gave the name "helium" to this gas, because of its solar 
origin. In 1895 Ramsay, while engaged in an analysis of 



The Sun's Sueroundings 79 

the mineral cleveite, discovered an unknown gas, which he 
found to give a yellow line near the position of D3. The 
spectroscope he employed was not powerful enough to deter- 
mine the position of the line with great accuracy, but Runge 
proved beyond a doubt, a short time later, that the line was 
actually in the position of Dg. However, he detected a faint 
companion to this line, on the side toward the red, which 
had never been observed in the solar prominences. An exam- 
ination of D3 in the prominence spectrum, made at the Ken- 
wood Observatory immediately upon the receipt of Runge's 
description of his laboratory results, showed the undoubted 
presence of a similar companion, which was found by repeated 
measures to agree well in position with Runge's determina- 
tions. The companion was so faint that it would easily escape 
observations made without knowledge of its existence. It 
may be said, however, that this first observation was greatly 
facilitated by the presence of a very bright prominence, in 
which D3 was beautifully shown. Huggins and others de- 
tected the duplicity of the line about the same time. 

As was anticipated by its behavior in the Sun, helium was 
found to be the lightest of all known gases, except hydrogen. 
Further study of the spectrum showed Dg to be only one of 
a series of lines, other members of which are also represented 
in the chromosphere. Many lines characteristic of the spectra 
of "Orion" stars, which had not been identified before Ram- 
say's discovery, are also due to helium. It is extremely prob- 
able that other new elements, not yet discovered on the Earth, 
are represented by some of the unknown lines of the solar 
spectrum. 

While the lines of hydrogen and helium are more bril- 
liant and conspicuous than all others in the visible spectrum 
of the chromosphere, it is nevertheless true that a very large 
number of lines due to other elements can be seen on any 
good day with a powerful telescope and spectroscope. The 



80 Stellar Evolution 

vapors of magnesium, iron, and several other substances are 
conspicuously represented by bright lines in the chromo- 
spheric spectrum; but these lines are shorter than those of 
hydrogen and helium, since the vapors do not rise to so great 
a height. With the Yerkes telescope it is even possible to 
observe a multitude of fine bright lines due to the vapor of 
carbon, which lies in close contact with the photosphere. 
The layer of carbon vapor is so thin that the least motion 
of the solar image, or a very slight disturbance of the atmos- 
phere, are sufficient to render the lines invisible. 

A total eclipse affords a most favorable opportunity to 
determine photographically the depths of the several layers. 
The simplest way of accomplishing this is to place a prism 
over the object-glass of a telescope, which is directed toward 
the Sun. When, at the moment of totality, the Moon covers 
the photosphere, arcs of the chromosphere are left projecting 
beyond the Moon's edge. After passing through the prism, 
the image formed on the photographic plate will appear like 
that reproduced in Plate XXXI, which was taken by Lord at 
the eclipse of 1900. The arcs represented here correspond 
to the various lines in the spectrum of the chromosphere. 
In this case, however, since no slit was used in the spectro- 
scope, the form of each arc is defined by the distribution of 
the corresponding vapor. If a prominence is present at any 
point, its image will be repeated in each of the arcs repre- 
senting the element it contains.^ Of course, this "spectrum 
of the flash," first observed by Young, and so called on account 
of its brief duration, can be photographed only during the few 
seconds while the Moon's edge is passing over the chromo- 
sphere. 

It will now be seen more clearly how the forms as well as 
the spectra of the prominences can be observed by the spec- 

1 The prominence group shown in Plate XXIX is faintly represented here 
(reversed in position) in each of the two stronger arcs. 



The Sun's Surroundings 81 

troscopic method without an eclipse. So long as a narrow 
slit is employed, the spectrum will consist of narrow lines, 
having the same form as the slit. That is, if the slit be 
straight, the lines will be short, straight sections of the 
chromosphere or prominences, corresponding in width to the 
slit. If the slit be curved, the lines will have a corresponding 
curvature. In other words, the lines are simply monochro- 
matic images of the slit. Hence, if the slit be widely opened, 
the lines will assume the form of that portion of the chromo- 
sphere or prominence which happens to lie across it. It is 
as though one were looking out through a narrow window 
upon a mass of great flames. 

The application of the spectroscopic method to the study 
of the chromosphere and prominences marked a new era in 
solar research. Daily observations were inaugurated with 
great enthusiasm by Lockyer, Young, Janssen, and other 
astronomers in Europe and America. It was found that the 
prominences could be divided into two classes — quiescent, 
or cloudlike, and eruptive. The former are much the more 
numerous, and may always be seen, in larger or smaller num- 
bers, at the Sun's limb. They change slowly in form, and 
sometimes persist for days, or until carried out of view by 
the solar rotation. When seen under excellent atmospheric 
conditions, the complex details of their structure resemble 
those of the clouds in our own atmosphere. The eruptive 
prominences change very rapidly in appearance, sometimes 
shooting up to elevations of over two hundred thousand miles 
in a few minutes (see Plates XXXV and XXXVI). Like the 
quiescent forms, they are most numerous at times of greatest 
sun-spot activity. They are never observed in very high 
latitudes, though the quiescent prominences appear at all 
parts of the solar circumference. The photographic study 
of these phenomena will be described in the next chapter. 



CHAPTER XI 
THE SPECTROHELIOGRAPH 

The spectroscopic method, as applied by astrophysicists 
in various parts of the world, has yielded a nearly continuous 
record of the solar prominences extending back over more 
than thirty years. For many purposes such a record is 
entirely satisfactory, and permits important conclusions to 
be drawn. But the process of observation is not only slow 
and painstaking : it is subject to the errors and uncertainties 
that necessarily attend the hand delineation of any object, 
seen through a fluctuating atmosphere. Moreover, changes 
in the forms of eruptive prominences are frequently so rapid 
that the draughtsman cannot record them. It was principally 
in the hope of simplifying the process of observation, and of 
rendering it more rapid and more accurate, that the spectro- 
heliograph was devised at the Kenwood Observatory in 
1889.^ 

The principle of this instrument is very simple. Its 
object is to build up on a photographic plate a picture of 
the solar flames, by recording side by side images of the 
bright spectral lines which characterize the luminous gases. 
In the first place, an image of the Sun is formed by a tele- 
scope on the slit of a spectroscope. The light of the Sun, after 
transmission through the spectroscope, is spread out into a 
long band of color, crossed by lines representing the various 
elements. At points where the slit of the spectroscope hap- 
pens to intersect a gaseous prominence, the bright lines of 
hydrogen and helium may be seen extending from the base 

1 It was subsequently learned that the method embodied in the spectrohelio- 
graph had been suggested by Janssen as early as 1869, reinvented by Braun of 
Kalocsa, and actually tried by Lohse at Potsdam. But it had not proved a success. 

82 



The Spectroheliograph 83 

of the prominence to its outer boundary. If a series of such 
lines, corresponding to different positions of the slit on the 
image of the prominence, were registered side by side on a 
photographic plate, it is obvious that they would give a rep- 
resentation of the form of the prominence itself. To accom- 
plish this result, it is necessary to cause the solar image to 
move at a uniform rate across the first slit of the spectro- 
scope, and, with the aid of a second slit (which occupies the 
place of the ordinary eyepiece of the spectroscope) , to isolate 
one of the lines, permitting the light from this line, and 
from no other portion of the spectrum, to pass through the 
second slit to a photographic plate. If the plate be moved 
at the same speed with which the solar image passes across 
the first slit, an image of the prominence will be recorded 
upon it. The principle of the instrument thus lies in photo- 
graphing the prominence through a narrow slit, from which 
all light is excluded except that which is characteristic of the 
prominence itself. It is evidently immaterial whether the 
solar image and photographic plate are moved with respect 
to the spectroheliograph slits, or the slits with respect to a 
fixed solar image and plate. 

This method, when first tried at the Harvard Observa- 
tory in 1890, proved unsuccessful. The lack of success 
was partly due to the fact that a line of hydrogen was 
employed. This line, though fairly suitable for the pho- 
tography of prominences with the perfected spectrohelio- 
graph of the present day, was too faint for successful use 
amidst the difficulties which surrounded the first experi- 
ments. Accordingly, when the work was resumed a year 
later at the Kenwood Observatory in Chicago (Plate XXXIII) 
an attempt was made, through a photographic investigation 
of the violet and ultra-violet regions of the prominence spec- 
trum, to discover other lines better fitted for future experi- 
ments. In the extreme violet region, in the midst of two 



84 Stellar Evolution 

broad dark bands which form the most striking feature of the 
solar spectrum, two bright lines (H and K) were found and 
attributed to the vapor of calcium. They had previously 
been seen visually by Young, but, on account of the insensi- 
tiveness of the eye for light of this color, they could not be 
observed satisfactorily. A careful study soon showed them 
to be present in every prominence examined, at elevations 
above the solar surface equaling or exceeding those attained 
by hydrogen itself (Plate XXXII, a). Their suitability for 
the purpose of prominence photography is due to several 
causes, among which may be mentioned their exceptional 
brilliancy, their presence at the center of broad dark bands 
which greatly diminish the brightness of the sky spectrum, 
and the comparatively high sensitiveness of photographic 
plates for light of this wave-length. 

While fairly efficient from an optical point of view, the 
spectroheliograph of the preceding year had possessed many 
mechanical defects. It sufficed to give photographs of 
individual prominences, but they were not very satisfactory. 
In a new instrument, devised for use with the 12-inch Ken- 
wood telescope, the principal defects were overcome, and 
means of securing the necessary conditions of the experi- 
ment were provided. The Kenwood spectroheliograph is 
shown in Plate XXXIV. In this instrument the solar image 
and photographic plate were fixed, while the first and second 
slits were made to move across them by means of a system 
of levers, set in motion by hydraulic power. The first trials 
of the instrument, made in January, 1892, were entirely 
successful, and the chromosphere and prominences surround- 
ing the Sun's disk were easily and rapidly recorded (Plates 
III, XXXV, and XXXVI). The details of their structure 
were shown with the sharpness and precision characteristic 
of the best eclipse photographs. And the opportunity for 
making such records, previously limited to the brief dura- 



The Spectroheliograph 85 

tion, never exceeding seven minutes, of a total eclipse, was 
at once indefinitely extended. Thus it became possible to 
study photographically the slowly varying forms of the qui- 
escent, cloudlike prominences, and, to particular advantage, 
the rapid changes of violent eruptions. 

But even before this primary purpose of the work had 
been accomplished, the possibility of making another and 
much more important application of the instrument had 
presented itself. A photographic study of the spectrum of 
various portions of the Sun's surface had shown the existence 
at many points of great clouds of calcium vapor, luminous 
enough to render their existence evident through the produc- 
tion of bright H and K lines on the solar disk (Plate XXXII, 
h and c). Some of these calcium clouds had, indeed, been 
known to exist through the important visual observations of 
Young, who had observed the bright H and K lines in the 
vicinity of sun-spots. But the vast extent and the charac- 
teristic forms of the phenomena could not be ascertained 
by such means. What was required was such a repre- 
sentation of the solar disk as the spectroheliograph had been 
designed to give in the case of the prominences. From a 
consideration of the results obtained in the spectroscopic 
study of the disk, it appeared probable that an important 
application of the spectroheliograph might be made in this 
new direction. 

Before describing this second application of the instru- 
ment, it may be well to recall the appearance of the Sun 
when seen with a telescope, or when photographed in the 
ordinary manner without a spectroheliograph. From photo- 
graphs like that reproduced in Plate II, we see that the 
most conspicuous features of the solar surface, at least so far 
as the eye can detect, are the well-known sun-spots. The 
bright faculae, which rise above the photosphere, are con- 
spicuous when near the edge of the Sun, but practically 



86 Stellae Evolution 

invisible when they liappen to lie near the center of the disk. 
The bright H and K lines, referred to in the last paragraph, 
were found in close association with the faculae, and it 
appeared probable that much of the highly heated calcium 
vapor, to which these bright lines are due, rises from the 
interior of the Sun through the faculae. It was therefore 
to be expected that a successful application of the spectro- 
heliograph to the photography of the luminous calcium 
clouds would give bright forms resembling those of the 
faculae. Furthermore, it was to be hoped that these brilliant 
clouds could be recorded, not only near the limb of the Sun, 
but also in the central part of the disk, since the bright 
reversals of the H and K lines were equally well photo- 
graphed in all parts of the image. 

The results of the first experiments, which were made at 
the beginning of 1892, were such as to justify fully the 
expectations that had been entertained. It was at once found 
possible to record the forms, not only of the brilliant clouds 
of calcium vapor associated with the faculae, and occurring 
in the vicinity of sun-spots, but also of a reticulated struc- 
ture extending over the entire surface of the Sun. The 
earliest use of the method was made in the study of the 
great sun-spot of February, 1892, which, through the great 
scale of the phenomena it exhibited and the rapid changes 
that resulted from its exceptional activity, afforded the very 
conditions required- to bring out the peculiar advantages of 
the spectroheliograph. In the systematic use of the instru- 
ment continued at the Kenwood Observatory through the 
following years, a great variety of solar phenomena were 
recorded, and the changes which they underwent from day 
to day— sometimes, in the more violent eruptions, from 
minute to minute — were registered in permanent form. 
During this period, which ended with the transfer of the 
Kenwood instruments to the Yerkes Observatory, over 3,000 



The Spectroheliograph 87 

photographs of solar phenomena were secured. From a 
systematic study of these negatives, in the course of which 
the heliographic latitude and longitude of the calcium clouds 
(subsequently named the flocculi) in many parts of the Sun's 
disk were measured from day to day (by Fox), a new deter- 
mination of the rate of the solar rotation in various latitudes 
has been made. This shows that the calcium flocculi, like 
the sun-spots, complete a rotation in much shorter time at the 
solar equator than at points nearer the poles. In other 
words, the Sun does not rotate as a solid body would do, but 
rather like a ball of vapor, subject to laws which are not yet 
understood. 

In this first period of its career the spectroheliograph 
had therefore permitted the accomplishment of two principal 
objects. It had provided a simple and accurate means of 
photographing the solar prominences in full sunlight, which 
gave results hardly inferior to those obtained during the 
brief moments of a total eclipse. It had also given a means 
of recording a new class of phenomena, known previously to 
exist only through glimpses of the bright calcium lines in the 
vicinity of sun-spots, but wholly invisible to observation, 
either visually or on photographs taken by ordinary methods. 
It was not difficult to see, however, that the possibilities of 
the new method were much greater than had been indicated 
by the work so far accomplished. It seemed probable that 
our knowledge of the finer details of the calcium flocculi 
would be greatly increased if provision could be made for 
photographing a much larger solar image with a spectro- 
heliograph of improved design. And it was furthermore 
evident that other applications of the instrument, involving 
the use of different spectral lines, and the employment of 
principles which had not been thoroughly tested in the 
earlier work, might reasonably be hoped for. Attempts 
were, indeed, made to photograph the Sun's disk with the 



Stellar Evolution 



dark lines of hydrogen, but the Kenwood spectroheliograph 
was not well adapted for this purpose. 

The 40-inch telescope of the Yerkes Observatory pro- 
vided the first requisite for the new work — namely, a large 
solar image, having a diameter of 7 inches as compared with 
the 2-inch image given by the Kenwood telescope. The 
construction of a spectroheliograph large enough to photo- 
graph such an image of the Sun involved serious difficulties, 
but these were finally overcome. The Rumford spectro- 
heliograph, designed to meet the special conditions of the 
new work, was built in the instrument shop of the Yerkes 
Observatory, and is now in daily use with the 40-inch 
telescope (Plate XXXVII). 

In this instrument the solar image is caused to move 
across the first slit by means of an electric motor, which 
gives the entire telescope a slow and uniform motion in 
declination. The sunlight, after passing through the first 
slit, is rendered parallel by a large lens at the lower end of 
the collimator tube. The parallel rays from this lens fall 
upon a silvered glass mirror, from which they are reflected to 
the first of two prisms, by which they are dispersed into a 
spectrum (Plate XLI, Fig. 1). After passing through the 
prisms, the light, which has now been deflected through an 
angle of 180°, falls upon a second large lens at the lower 
end of the camera tube. This forms an image of the 
spectrum at the upper end of the tube, where the second 
slit is placed. Any line in the spectrum may be made to 
fall upon this slit, by properly adjusting the mirror and 
prisms. Above the slit, and nearly in contact with it, the 
photographic plate is mounted in a carriage, which runs on 
rails at right angles to the length of the slit. The rails are 
covered by a light-tight camera box, so that no light can 
reach the plate except that which passes through the second 
slit. While the solar image is moving across the first slit. 



The Spectroheliograph 89 

the plate is moved at the same rate across the second slit, by 
a shaft leading down the tube from the electric motor, and 
connected, by means of belting, with screws that drive the 
plate-carriage. 

Photographs of the solar disk taken with this instrument 
under good atmospheric conditions reveal a multiplicity of fine 
details (Plate XXXVIII). The entire surface of the Sun is 
shown by these plates to be covered by minute luminous 
clouds of calcium vapor, only about a second of arc in 
diameter, separated by darker spaces, and closely resembling 
in appearance the well-known granulation of the solar photo- 
sphere (Plate XXXIX) . A sharp distinction must, however, 
be drawn between this appearance, which is wholly invisible to 
the eye at the telescope, and the granulation of the photosphere. 
In accordance with Langley's view, the grains into which the 
solar surface is resolved under good conditions of visual obser- 
vation are the extremities of columns of vapor rising from the 
Sun's interior. They seem to mark the regions at which 
convection currents, proceeding from within the Sun, bring 
up highly heated vapors to a height where the temperature 
becomes low enough to permit them to condense. It might 
be anticipated that out of the summits of these condensed 
columns, other vapors, less easily condensed, would continue 
to rise, and that the granulated appearance obtained with 
the spectroheliograph may represent the calcium clouds 
thus ascending from the columns (Plate XL). We might, 
indeed, go a step farther, and imagine the larger and higher 
calcium clouds to be constituted of similar vaporous columns, 
passing upward through the chromosphere, and perhaps at 
times extending out into the prominences themselves. A 
means of research now to be described, which represents 
another application of the spectroheliograph, involving a 
new principle, seems competent to throw some light on this 
question. 



90 Stellar Evolution 

Mention has already been made of the faculae, which are 
simply regions in the photosphere that rise above the ordi- 
nary level. Near the edge of the Sun their summits lie 
above the lower and denser part of that absorbing atmos- 
phere which so greatly reduces the Sun's light near the 
limb, and in this region the faculae may be seen visually. 
At times they may be traced to considerable distances from 
the limb, but as a rule they are inconspicuous or wholly 
invisible toward the central part of the solar disk. The 
Kenwood experiments had shown that the calcium vapor 
coincides closely in form and position with the faculae, and 
hence the calcium clouds were long spoken of under this 
name. In the new work at the Yerkes Observatory the dif- 
ferences between the calcium clouds and the underlying 
faculae became so marked that a distinctive name for the 
vaporous clouds appeared necessary. They were therefore 
designated Jlocculi, a name chosen without reference to their 
particular nature, but suggested by the flocculent appearance 
of the photographs. 

In order to analyze these flocculi and to determine their 
true structure, a method was desired which would permit 
sections of them at different heights above the photosphere to 
be photographed. Fortunately there is a simple means (first 
described by Deslandres) which appears to accomplish this 
apparently difficult object. At the base of the flocculi the 
calcium vapor, just rising from the Sun's interior, is com- 
paratively dense. As it passes upward through the flocculi 
it reaches a region of much lower pressure, and during the 
ascent it might be expected to expand, and therefore to 
become less dense. Now we know from experiments in the 
laboratory that dense calcium vapor produces very broad 
spectral bands, and that, as the density of the vapor is 
decreased, these bands narrow down into fine, sharp lines 
(Plate XLI, Fig. 2). An examination of the solar spectrum 



The Spectroheliograph 91 

will show that the H and K lines of calcium give evidence of 
the occurrence of this substance under widely different densi- 
ties in the Sun. The broad dark bands, which for convenience 
we designate Hj and Kj, are due to the low-lying, dense 
calcium vapor (Plate XXXII). At their middle points (over 
flocculi) are seen two bright lines, which are much narrower 
and better defined. These lines, designated H, and K.,, are 
the ones ordinarily employed in photographing the flocculi 
with the spectroheliograph. Superposed upon these bright 
lines are still narrower dark lines, due to the absorption of 
cooler calcium vapor at higher elevations (Hg, Kg). It will 
be seen that the evidence of the existence of calcium vapor 
at various densities in the Sun is apparently complete, and 
that we may here find a way of photographing the vapor at 
low levels without admitting to the photographic plate any 
light that comes from the rarer vapors at higher levels. It is 
simply necessary to set the second slit of the spectrohelio- 
graph near the edge of the broad Hj or Kj bands, in order 
to obtain a picture showing only that vapor which is dense 
enough to produce a band of width sufficient to reach this 
position of the slit. No light from the rarer vapors above 
can enter the second slit under these circumstances, since 
they are incapable of producing a band of the necessary 
width.' 

The great sun-spot of October, 1903, afforded an oppor- 
tunity to try this method in a very satisfactory manner. 
Sections of the calcium vapor in the neighborhood of this 
spot-group, corresponding to the two different levels photo- 
graphed on October 9, are shown in Figs. 1 and 2, Plate 

1 The bright regions photographed in this way resemble the faculae very closely, 
and may be regarded as essentially identical with them, since the white light from 
the continuous spectrum of the faculae contributes in an important degree to the 
formation of the photographic images. However, any dense calcium vapor which 
extends beyond the boundaries of the faculae will be recorded on the photograph. 
In any case we should expect the dense calcium vapor, supposed to be rising from 
the faculae, to correspond closely with them in form. 



92 Stellak Evolution 

XLII.^ The manner in which the vapor at the Hg level over- 
hangs the edge of the sun-spot is very striking, and thorough 
study should throw some light on the conditions which exist 
in such regions. For it is possible, not only to photograph 
sections of the vapor at various levels, but also to ascertain, 
by the displacement of the H2 or H3 line, as photographed by 
a powerful spectrograph , the direction and velocity of motion 
of the vapor which constitutes the flocculi. It is commonly 
found that the vapor is moving upward at the rate of about 
one kilometer per second, though the velocity varies con- 
siderably at different points and under different conditions. 
The photographs occasionally show the existence of flocculi 
remarkable for their great brilliancy. In these regions active 
eruptions are in progress. The vapor, rendered highly 
luminous by intense heat or other causes, is shot out from 
the Sun's interior with great velocity. Consequently there 
are rapid changes in the forms of these brilliant regions, 
whereas the ordinary flocculi change slowly, and represent 
a much less highly disturbed condition of affairs. The 
brilliant eruptive flocculi always occur in active regions of 
the solar surface, and probably correspond with the erup- 
tive prominences sometimes photographed projecting from 
the Sun's limb. A remarkable instance was recorded on 
the Kenwood photographs, which showed four successive 
stages of an eruption of calcium vapor on an enormous scale. 
A vast cloud thrown out from the Sun's interior completely 
blotted from view a large sun-spot, and spread out in a few 
minutes so as to cover an area of four hundred millions of 
square miles. 

1 Although these photographs have been arranged for comparison with the 
stereoscope, it is to be understood that no stereoscopic eiJect in the ordinary sense 
will be obtained in examining them. The purpose of using the stereoscope is simply 
to allow the images to be superposed, thus permitting them to be seen at the same 
point in rapid succession by moving a card so as to cover alternately the two lenses 
of the stereoscope. Thus the manner in which the calcium flocculi overhang the 
penumbra, and sometimes the umbra, of spots can be observed. 



The Specteoheliograph 93 

Although the eruptive flocculi probably correspond in 
many instances with eruptive prominences, it must not be 
concluded that the quiescent calcium flocculi correspond 
with the quiescent, cloudlike prominences. As a matter of 
fact, we have good evidence for the belief that the flocculi 
shown in these photographs represent in most instances 
comparatively low-lying vapors, while the prominences, 
which extend above the level of the chromosphere, do not 
ordinarily reveal themselves as bright objects in projection 
against the disk. 

So far, we have considered the photography of the Sun 
with the light of the H and K lines of calcium. But it 
must naturally occur to anyone familiar with the solar 
spectrum that it should be possible to take photographs 
corresponding to other lines, and thus representing the 
vapors of other substances. For the darkness of the lines 
is only relative; if they could be seen apart from the bright 
background of continuous spectrum on which they lie, these 
lines would shine with great brilliancy. It is thus evident 
that, if all light except that which comes from one of these 
dark lines can be excluded from the photographic plate by 
means of the second slit of the spectroheliograph, it should be 
possible to obtain a photograph showing the distribution of 
the vapors corresponding to the line in question. 

At this point attention should be called to the extreme 
sensitiveness of the spectroheliograph in recording minute 
variations in the intensity of a line — variations so slight that 
no trace of them can be seen in a spectrum photograph 
showing only the line itself. A well-known physiological 
effect is here concerned, for it is common experience that 
the eye cannot detect minute differences of intensity in 
various parts of an extremely narrow line, whereas these 
would become conspicuous if the line were widened out 
into a band of considerable width. The spectroheliograph 



94 Stellar Evolution 

records side by side upon the photographic plate a great 
number of images of a line which, taken together, build up 
the form of the region from which the light proceeds. In 
this way the full benefit of the physiological principle is 
derived, and very minute differences of intensity at various 
parts of the solar disk are clearly registered upon the 
plate. 

It is obviously essential in photographing with the dark 
lines to exclude completely the light from the continuous 
spectrum on either side of the line employed. The admis- 
sion of even a small quantity of this light might completely 
nullify the slight differences of intensity recorded by the 
aid of the comparatively faint light of the dark line. As 
the second slit cannot be narrowed beyond a certain point, 
it is evident that for successful photography with the dark 
lines their width must be increased by dispersion in the 
spectroheliograph to such a degree as to make them wider 
than the second slit. 

The first satisfactory photographs obtained with dark lines 
were made with the Rumford spectroheliograph in May, 1903. 
The lines of hydrogen were chosen for this purpose, on 
account of their considerable breadth, and because of the 
prominent part played by this gas in the chromosphere and 
prominences. In order to secure sufficient width of the 
lines, the mirror of the spectroheliograph was replaced by a 
large plane grating having 20,000 lines to the inch. After 
leaving the grating the diffracted light enters the prisms, 
where it is still further dispersed before the image of the 
spectrum is formed upon the second slit. The effect of the 
prisms is not only to give additional dispersion, but also to 
reduce the intensity of the diffuse light from the grating — 
a most important matter in work of this nature. The hydro- 
gen lines employed were H^, Hy, or i?5, in the green-blue, 
blue, and violet, respectively. 



The Spectroheliograph 95 

On developing the first plate it was surprising to find 
evidences of a mottled structure covering the Sun's disk, 
resembling in a general way the structure of the calcium 
flocculi, but differing in the important fact that, whereas the 
calcium flocculi are bright, those of hydrogen are dark 
(Plate XLIII). This result was confirmed by subsequent 
photographs, and it was found that in general the hydrogen 
flocculi are dark, although in certain disturbed regions bright 
hydrogen flocculi appear. Some of these are eruptive in 
character, and correspond closely with the brilliant eruptive 
calcium flocculi. But in other cases, in regions where no 
violent eruptive disturbances seem to be present, the hydrogen 
flocculi frequently appear bright instead of dark (Plate 
LXXII). Such regions are usually in the immediate vicinity 
of active sun-spots, where it is probable that the temperature 
of the hydrogen is considerably higher than in the surround- 
ing regions. Since a higher temperature would undoubtedly 
produce increased brightness, the spectroheliograph thus 
seems to afford a method of distinguishing between regions 
of higher and lower temperature — an additional property 
which should prove of great value in investigations on the 
vapors associated with sun-spots. It is possible, of course, 
that the increased brightness is due, not merely to an 
increase of temperature, but to other causes, perhaps of a 
chemical or electrical nature, which are not yet understood. 
But the assumption that increased temperature is the effective 
cause may be provisionally accepted as very probable. 

The comparative darkness of the ordinary hydrogen flocculi 
evidently indicates that this gas in the flocculi for some rea- 
son radiates less light than the hydrogen gas which , probably 
after diffusing from the flocculi, has spread in a nearly uni- 
form mass over the entire surface of the Sun. The simplest 
hypothesis is to assume that the diminished brightness of 
the flocculi is due to the reduced temperature in the upper 



96 Stellar Evolution 

chromosphere, where the absorption probably occurs. The 
results of work at Mount Wilson, described in chap, xvi, seem 
to render this view probable. It should be emphasized at 
this point, however, that the explanation of spectroheliograph 
results offered in this chapter is merely an hypothesis, which 
subsequent investigation may not prove to be correct. 
According to Julius, the fiocculi are not luminous clouds, but 
the effects of anomalous dispersion of light passing out from 
the Sun's interior through vapors of unequal density (see 
p. 148). 

The Rumford spectroheliograph was also used to secure 
photographs with some of the stronger dark lines of iron 
and other substances. But even with the grating the disper- 
sion was insufficient to give thoroughly trustworthy results, 
except in a very few cases. It was evident that much greater 
dispersion must be employed in order to realize the full advan- 
tages of the method in future work. Subsequent progress 
in the development of the spectroheliograph is described in 
chap. xvi. 

Within a short time after the first work at the Kenwood 
Observatory the spectroheliograph came into general use. 
Evershed constructed and successfully used one of these 
instruments in England, and a year later Deslandres, whose 
admirable work on the spectra of the fiocculi was contempo- 
raneous with the investigations at the Kenwood Observatory, 
undertook systematic research with a spectroheliograph at 
the Paris Observatory. His contributions to the develop- 
ment of the instrument have been very valuable. Other 
spectroheliographs are now used daily in India, Sicily, Spain, 
Germany, England, and the United States. 



CHAPTER XII 

THE YERKES OBSERVATORY 

The formulation of the theory of natural selection by 
Darwin was the result of an extensive series of closely cor- 
related investigations, covering a broad field. His object 
was not merely to bring together a great collection of plants 
or animals, describe their peculiarities, and confer upon them 
appropriate names. To Darwin each of these plants and 
animals might be of great interest. But brilliant plumage, 
unusual form, and other distinctive peculiarities were of 
importance to him mainly because of their bearing upon the 
question of development, or the possible relationship of the 
particular specimen to others. It is obvious that a study of 
such relationships must greatly enhance, rather than dimin- 
ish, the interest of the investigator in the peculiarities which 
distinguish species. Having in mind a governing principle, 
he may detect, through the aid of delicate markings or minute 
modifications of form which might otherwise be inappre- 
ciable, the evidences of development which constitute the 
prime object of his search. 

Similar tendencies toward unification and correlation have 
shown themselves in every department of science. Co-opera- 
tive undertakings on a large scale, which have enlisted the 
best efforts of scientific men in all parts of the world, are 
common at the present time. It may confidently be pre- 
dicted that the future will see such work greatly extended, 
and that the various agencies which can thus be employed 
to advance science will be utilized in an increasingly effec- 
tive manner. 

In astronomical and astrophysical research the opportu- 

97 



Stellae Evolution 



nities for co-operation and correlation are unusually good, 
and have yielded many important results. The impossibility 
of completing at any one observatory the extensive investi- 
gations required for the solution of large cosmical problems, 
and the advantages which may result from the discussion of 
observations made simultaneously or at stated intervals from 
stations differing widely in geographic position, altitude, or 
climatic conditions, render co-operation essential in many 
cases. Plans for international co-operation in solar research 
are mentioned elsewhere. An attempt to provide for the 
closest possible correlation of work within a single observa- 
tory is also described in this book. 

In establishing an observatory, either one of two policies, 
both represented in existing institutions, may be adopted. 
On the one hand, attention may be directed to the prosecu- 
tion of individual researches or extensive routine investiga- 
tions, not necessarily closely related to one another, but each 
constituting an important contribution to knowledge. On 
the other hand, a single large problem may be chosen, and 
all individual investigations planned so as to lead as directly 
as possible toward its solution. The observations required 
may be very diverse, and cover a broad field. Each, how- 
ever, to be most effective for its purpose, must be chosen with 
special reference to the existing needs, and the general pro- 
gramme must be revised from time to time, in the light of 
every important advance. 

The Yerkes Observatory may serve as an example of an 
institution in which extensive individual investigations, 
differing widely in character, comprise the programme of 
research.' Its scheme of work was based on a deliberate 
intention to realize the fullest possible advantages of the 
40-inch refractor in the diverse researches for which it is 

1 In the astrophysical work, however, an effort was made to correlate the solar, 
stellar, and laboratory investigations. 



The Yerkes Observatory 99 

peculiarly adapted. The object of the Mount Wilson Solar 
Observatory of the Carnegie Institution, however, is to con- 
centrate its entire attention upon the study of the Sun and 
the problem of stellar evolution. 

After the spectroheliograph had been tested at the Ken- 
wood Observatory, it seemed certain that this method was 
capable of further extension, and the desirability of securing 
better instrumental facilities accordingly presented itself. 
The establishment of the new University of Chicago appeared 
to offer the best prospects in this direction. The opportunity 
of purchasing two disks of glass for the objective of a 
40-inch refractor was encountered in 1893. This glass had 
been ordered three years before for a telescope to be erected 
on Mount Wilson in southern California^ — an odd coinci- 
dence in the light of subsequent events. As funds were not 
available for the completion of the California project the 
glass disks, then in the hands of Alvan Clark & Sons, were 
obtainable. The opportunity was an unusual one, since the 
disks were of the largest size and of the most perfect optical 
glass. After several unsuccessful attempts to secure the 
funds from other sources, the matter was placed before Mr. 
Charles T. Yerkes by President Harper. He promptly sig- 
nified his desire to provide for the construction of a 40-inch 
refractor. The glass was purchased, a contract arranged 
with Clark to complete the object-glass, and the mounting 
ordered from Warner & Swasey. The construction of the 
Yerkes Observatory was undertaken in 1895 and completed 
in 1897. 

The gift which provided for the Yerkes Observatory was 
made before the University of Chicago had opened its doors 
to students. In fact, the original idea of establishing a col- 
lege, rather than a university, had hardly been outgrown, 
and the question of the recognition to be accorded to research 
was still a cause of concern to the members of the rapidly 



100 Stellar Evolution 

enlarging faculty. A narrow view of the future on the part 
of the trustees might have led to the erection of the obser- 
vatory in Chicago, and its use for the purposes of instruction 
rather than for those of research. Fortunately, a different 
policy prevailed. It was recognized that the 40-inch tele- 
scope should be exclusively devoted to investigation, and that 
a site in the immediate neighborhood of the university 
grounds would prevent its effective use. It was accord- 
ingly decided to secure a site in the most favorable location 
within a reasonable distance of Chicago, and a tract of 
land in Wisconsin, on the shore of Lake Geneva, was finally 
selected. 

The plan of the building shows the influence of the Lick 
Observatory and the Astrophysical Observatory of Potsdam, 
both of which embody many admirable features. The adopted 
form of a Roman cross permitted the three domes to be sepa- 
rated to such an extent that they practically do not interfere 
in the least with one another (Plate XLIV). The desire of 
the donor for an ornate structure, and the decision of the 
architect to introduce rather florid embellishments of terra- 
cotta, led to the use of brick as a building material. This was 
quite in accordance with convention, but in conflict with the 
condition, well known to astronomers, that the temperature 
within an observing-room should be as nearly as possible the 
same as the temperature of the outer air. The massive brick 
wall of the great tower in which the 4:0-inch telescope is 
mounted is therefore decidedly inferior to a light steel con- 
struction, with a thin metallic wall, shielded from the Sun by 
an outer wall of similar type. Architectural considerations, 
however, have weighed as heavily in nearly all of the world's 
largest observatories, and the complete freedom of action, 
subsequently experienced at Mount Wilson, had not yet been 
attained. 

The engineering problems presented by the great size 



\ 

The Yerkes Obseevatory 101 

of the Yerkes telescope, and of the dome under which it was 
mounted, were such as to tax the efforts of even so skilful a 
firm as that of Warner & Swasey, to whom the work was 
intrusted. The admirable qualities of the mounting of the 
Yerkes telescope show the advantage of the experience gained 
by them in constructing the Lick telescope. The dome and 
rising-floor, after several faults of design and construction 
had been remedied, also performed very well. Thoroughly 
tested by continuous use, by night and by day, for a period of 
ten years, the entire plant may certainly be considered to 
reflect much credit upon these well-known engineers. 

The 4:0-inch telescope, and other instruments of the 
Yerkes Observatory, have already been described in previous 
chapters, but a few additional details may be of interest. 
The object-glass, which was put in place only a few weeks 
before the death of Alvan G. Clark, the last member of the 
celebrated firm of Alvan Clark & Sons, is made up of two 
lenses. The outer lens, made of crown glass, is double con- 
vex in form (Plate XLV). The inner lens, separated from 
the other by a distance of about eight inches, is plano-con- 
cave, and made of flint glass. The total weight of the glass 
in the two lenses is about 500 pounds. The rough glass 
disks, from which the lenses were fashioned by the Clarks, 
were made by Mantois, of Paris. The glass is of extra- 
ordinary purity and transparency, but in spite of this fact it 
absorbs much light, on account of its considerable thickness 
(about three inches in all). The conditions are very dif- 
ferent from those of a reflecting telescope, where much less 
perfect glass is required, since in the latter case the light is 
reflected from a layer of pure silver on the front surface and 
therefore suffers no absorption in transmission (though some 
light is lost in reflection). It has already been pointed 
out that refracting and reflecting telescopes have their own 
peculiar advantages and defects. The choice of the one or 



102 Stellae Evolution 

the other must depend upon the needs of the work for which 
it is required. 

In order to direct the 40-inch telescope to a faint star, 
the sidereal time, as well as the right ascension and declina- 
tion of the star, must be known. After the opening in the 
dome has been turned toward the proper quarter of the 
heavens, the telescope is moved in right ascension (i. e., 
around the polar axis, which is parallel to the Earth's axis) 
until the hour circle, attached to this axis, indicates the 
proper reading. This reading is determined by taking the 
difference between the sidereal time and the right ascension 
of the star. The result gives the distance of the star from 
the meridian, expressed in hours and minutes of time. The 
motion of the telescope in right ascension is produced by 
means of an electric motor, controlled by a rope running down 
the north face of the iron column and easily reached from 
the rising-floor. The next operation is to move the telescope 
in declination (i. e., around an axis at right angles to the 
polar axis) until the declination circle indicates the proper 
reading, so many degrees north or south of the equator. If 
the eye-end of the telescope is then too high to be reached 
by the observer on the rising-floor, the floor is raised by 
means of an electric motor, controlled by a switch near the 
telescope column. An adjoining switch controls the motor 
which turns the dome. On looking into the eye-piece the 
star will be found in the field, provided the setting has been 
accurately made. The telescope is next clamped in right 
ascension and declination. It will then be carried by the 
driving-clock, which causes the polar axis to rotate through 
a complete revolution in twenty-four hours. The apparent 
motion of the star in the heavens is thus counteracted, and 
the image remains fixed in the field of view, where it may 
be studied in any way desired. 

If, for example, the observer wishes to measure the posi- 



The Yerkes Observatory 103 

tion of the star with respect to other stars in its neighbor- 
hood, this is accomplished by means of a position micrometer, 
in which a fine spider line can be moved through the neces- 
sary distance by a micrometer screw. The value of one divi- 
sion of the micrometer head, in seconds of arc, is previously 
determined by measuring the distance between two known 
stars, w^hose positions have been accurately fixed by means 
of a meridian circle. Burnham's admirable observations 
of double stars with the 40-inch telescope have all involved 
the accurate micrometric measurement of the distance sepa- 
rating the stars of each pair. The position angle of the line 
joining the two stars, with reference to a north-and-south 
line in the heavens, is also measured in each case with the 
aid of a divided circle attached to the micrometer. On 
account of the large aperture of the telescope, it is possible 
to separate with it stars about one-tenth of a second of arc 
apart, provided the atmospheric conditions are sufficiently 
good for the purpose. As the distance between the two 
images in the principal focus of the telescope would, in this 
case, amount to but little over one three-thousandth part of 
an inch, it is obvious that the best of conditions are required 
for such exacting work. 

Barnard's observations with the Yerkes telescope have also 
involved the constant use of the micrometer. The difficulty of 
the work, and the patience required to pursue it, can be ima- 
gined when it is remembered that Barnard has measured the 
positions of hundreds of stars in such a closely crowded cluster 
as that illustrated in Plate XIX. In such work as this the 
observer remains standing throughout the entire night. It 
should also be remembered that in the open dome the tem- 
perature sometimes falls to — 20" F. in the rigorous Wiscon- 
sin winters. It is evident that only the greatest interest and 
devotion on the part of the observer can permit him to make 
accurate measures, night after night, under such conditions. 



104 Stellar Evolution 

We have already seen (in chap, xi) how the Rumford 
spectroheliograph is used with the Yerkes telescope. As the 
spectroheliograph weighs about 700 pounds, and must be 
attached each morning and taken off at night, special arrange- 
ments are required to facilitate this work. Each heavy 
instrument used in conjunction with the telescope is mounted 
on a carriage, which stands on the rising-floor. When the 
change is to be made from one attachment to another, the 
floor is raised to its highest position and the telescope tube 
firmly anchored to it by means of a steel bar. This is to 
obviate any danger of accident when the balance of the tube 
is temporarily disturbed. The carriage bearing the spectro- 
heliograph is brought to the eye-end of the telescope, the 
spectroheliograph clamped to its supporting ring, and over 
700 pounds of iron weights removed from the telescope 
tube. This restores the balance, which must be adjusted 
to a nicety. 

The Bruce spectrograph (Plates XL VI and LXXVIII) 
is used by Frost for the photographic study of stellar spectra. 
The image of a star is formed on the slit of the spectrograph, 
which is about one-thousandth of an inch in width. The 
light then passes to a collimator lens, which renders the rays 
parallel. Three large prisms, next traversed by the rays, 
bend them through an angle of 180° and disperse them 
into a spectrum. The camera, lens forms an image of the 
spectrum upon the photographic plate. Throughout the 
exposure, which may be continued several hours, the ob- 
server watches the star image and keeps it accurately on 
the slit, any imperfections in the driving of the telescope 
being corrected by means of electric slow motions. In order 
to eliminate the effect of the changing temperature in the 
open dome, the spectrograph is inclosed in a tight-fitting 
case, the interior of which is maintained at a uniform tem- 
perature by electric-heating coils. 



The Yekkes Observatory 105 

In order to determine the position of the lines in a 
spectrum, a suitable comparison spectrum is required. This 
is obtained by passing an electric spark between poles of 
titanium or iron and photographing the spectrum of the 
spark on each side of that of the star. An enlargement of 
one of Frost and Adams' photographs of ?; Leonis, made in 
this way, is reproduced in Plate XLVII. It will be seen that 
the lines of the comparison spectrum are shifted a slight 
distance toward the red (right), with reference to the corre- 
sponding lines in the star. This shift is due to the motion 
of the star away from the Earth, which in this instance 
amounts to 28 kilometers per second. On account of its 
orbital motion, the Earth was moving toward the star on 
this date at the rate of 26 kilometers per second. Hence the 
velocity of y Leonis with respect to the Sun was -f 2 kilo- 
meters per second. 

Such displacements of the lines provide the only means 
of determining whether a star is approaching or receding 
from the Earth. This method, first tried visually by Hug- 
gins, was successfully adopted for photographic work by 
Vogel, and subsequently greatly refined by Campbell, who 
applied it with remarkable success at the Lick Observatory. 
In the hands of Campbell, Frost, and others, it has resulted in 
the discovery of many "spectroscopic binaries" — double stars 
in which the component members are revolving at such great 
velocities that they periodically displace the lines in their 
spectra. In most of these binaries one of the components 
is a dark star. Our only clue to their duplicity is thus fur- 
nished by the fact that the lines move back and forth with 
respect to the comparison lines, the displacement being 
toward the violet when the star is approaching, and toward 
the red when it is receding from the Earth. In a subsequent 
chapter it will appear how photographs of stellar spectra 
are used in the study of stellar evolution. 



106 Stellar Evolution 

The Rumford spectrolieliograph and the Bruce spectro- 
graph were constructed in the instrument shop of the Yerkes 
Observatory. It had long been customary for observatories 
to provide means of repairing their own instruments, but the 
work of construction had, as a rule, been left to the profes- 
sional instrument-makers. At the Yerkes Observatory a well- 
equipped shop was not only a convenience, but a necessity. 
The funds given for the establishment of the observatory did 
not provide for a general equipment of minor instruments. 
In the absence of the means of purchasing instruments, the 
only alternative was to construct them. Fortunately, a 
number of machine tools had formed part of the equipment 
of the Kenwood Observatory and were immediately available. 
The appropriations of the University of Chicago permitted a 
skilled instrument-maker to be regularly employed, and spe- 
cial gifts, received from various sources in subsequent years, 
sometimes enabled us to keep several men at work. The 
instrument shop, at first under the direction of Wadsworth 
and subsequently under Ritchey (who was in charge of the 
optical shop from the beginning), proved to be indispensable 
to the success of the Observatory's work. Not only the instru- 
ments already mentioned, but also the 2-foot reflector, the 
Snow telescope, a 3^-inch transit instrument, spectroscopic 
and other apparatus used in the laboratory, and many special 
instruments and appliances employed with the 40-inch tele- 
scope and in other departments of the work, came from this 
source. It may be said that in a large astrophysical observa- 
tory, where new types of instruments are constantly being 
devised, a well-equipped instrument shop is essential if the 
best results are to be obtained. This is largely because of 
the advantage of having the instruments constructed under 
the immediate supervision of the men who are responsible for 
their design. 

The optical shop was another feature of the Yerkes 



The Yerkes Observatory 107 

Observatory which contributed in a most important manner 
to its work. Here Ritchey made numerous mirrors — j)lane, 
concave, and convex — for use in the Snow telescope, the 
2-foot reflector, and other instruments, and here also he did 
a large part of the work on the 60-inch mirror, which was 
subsequently transferred to the Solar Observatory. As the 
methods employed in grinding and polishing this mirror are 
described in chap, xxiii, no further mention will be made of 
them here. It may be said, however, that many special 
investigations set on foot at the Yerkes Observatory could 
not have been undertaken without the unique advantages 
afforded by the optical shop. 

Still another feature of the Yerkes Observatory, which 
was subsequently repeated, in improved form, at Mount 
Wilson, is the spectroscopic laboratory, in which various 
solar and stellar phenomena are imitated experimentally. 
Apparatus for producing sparks between metallic poles in 
air, in liquids, and in compressed gases is arranged on the 
circumference of a circular table. Low-voltage arcs are also 
provided, the purpose of the equipment being to furnish 
means of varying, between wide limits, the conditions of 
temperature and pressure, and of gaseous or liquid environ- 
ment, in which the metallic vapors emit their characteristic 
radiations. By setting at the proper angle a plane mirror, 
mounted at the center of the table, light from any source 
can be reflected to a concave mirror, which forms an image of 
the source on the slit of a large concave grating spectrograph. 
The most extensive single investigation made in this labora- 
tory was a study of the spectrum of the spark in liquids and 
compressed gases, to test Wilsing's pressure theory of 
temporary stars. 

In the diversified work of the Yerkes Observatory the 
desire to attack the problem of stellar evolution in the most 
effective manner was not forgotten. Experience with the 



108 Stellae Evolution 

large concave grating of the Kenwood Observatory had 
furnished convincing evidence of the advantages of fixed 
instruments mounted on piers, and the beautiful resolution of 
the solar spectrum with this apparatus made observations of 
stellar spectra with small prism spectroscopes seem unsatis- 
factory. It was felt from the first that every effort should be 
made to devise a telescope capable of bringing a large and 
well-defined solar image, or a sharp and brilliant stellar 
image, into a laboratory, where it could be observed to the 
best possible advantage, with appliances too large or too 
heavy for use with moving telescopes. It seemed clear that, 
if this desire could be realized, and if the full advantages 
of reflecting telescopes for astrophysical research could be 
attained, the means thus provided should render possible a 
well-directed attack on the problem in mind. 

The work of the Rumford spectroheliograph showed that 
the further development of this instrument must involve a 
considerable increase in dispersion, so as to permit the use 
of the narrower dark lines. This meant an instrument of 
large dimensions, necessarily to be mounted in a fixed posi- 
tion, since it could not be attached to a moving telescope 
tube. Another piece of work pointed to the same require- 
ment. At the Kenwood Observatory attempts were made to 
photograph the spectra of sun-spots, and negatives were 
secured showing a few of the more conspicuous widened lines. 
The need of a larger solar image for this work was met by 
the Yerkes telescope. A marked improvement in the photo- 
graphs resulted. However, it was clear that photographs of 
spot spectra suitable for the most refined investigations could 
not be obtained without the use of a spectrograph of much 
higher dispersion. For satisfactory results a spectrograph 
of at least 10 feet focal length was needed, and this could 
not be attached to the moving telescope tube. Here, again, 
was another argument for the fixed type of telescope. 



The Yekkes Observatory 109 

The work of constructing such an instrument was accord- 
ingly taken up. The original purpose of building a heliostat 
was modified, through the recognition of the superior advan- 
tages of the coelostat, introduced by Turner for eclipse 
observations. A 30-inch coelostat, designed by Ritchey, 
was constructed in the instrument shop of the Yerkes 
Observatory. This was destroyed by fire, but a gift from 
Miss Snow of Chicago, in memory of her father, provided 
the funds required for the Snow telescope. In the prelimi- 
nary tests of this instrument at the Yerkes Observatory the 
images were not very satisfactory, but it subsequently gave 
admirable results at Mount Wilson. 

In establishing the Carnegie Institution at Washington, 
Mr. Carnegie gave expression to his appreciation of the fact 
that some of the most fundamental needs of scientific research 
could not be supplied by existing agencies. As a rule, a 
university must build its observatory or biological laboratory 
near at hand, rather than at a site chosen because of atmos- 
pheric advantages or the richness of the local fauna and flora. 
Its funds, usually given for specific purposes, are likely to be 
unavailable, or perhaps inadequate, to provide a sufficiently 
large corps of investigators, devoted to research. If, through 
the efforts of one of its facnlty, a new and promising instru- 
ment is projected, the trustees may not be in a position to 
supply the financial means required to construct it. Such 
conditions result from the very nature of a university's 
work, and consequently affect, in some degree, the policy 
of even so progressive an institution as the University of 
Chicago, where the authorities strongly favor original in- 
vestigation. The Carnegie Institution, devoted exclusively 
to the furtherance of research, is not thus hampered. It 
therefore came about that this new Institution, with the 
cordial co-operation of the University of Chicago, made pro- 
vision for the continuation and development of the work set 



110 Stellae Evolution 

on foot at the Kenwood and Yerkes Observatories. A com- 
mittee, appointed to report on the advisability of establishing 
an observatory for solar research, and another observatory for 
observations of the southern heavens, favored both of these 
projects. A careful test of various sites in the United States 
and in Australia, made at the request of the committee by 
Hussey, led to the provisional selection of Mount Wilson 
(5,886 feet), near Pasadena in southern California, as the 
site for the proposed solar observatory. An appropriation, 
granted by the Carnegie Institution in 1904, furnished the 
means of sending an expedition from the Yerkes Observatory 
to Mount Wilson. The Snow telescope was erected on the 
mountain, in a new type of house especially designed for it. 
An instrument shop was established in Pasadena for the 
construction of the spectroheliographs and other apparatus 
required for use with the Snow telescope. In December, 
1904, the Carnegie Institution decided to establish a solar 
observatory of its own on Mount Wilson. Through the 
courtesy of the authorities of the Yerkes Observatory and 
the University of Chicago, the Snow telescope was retained 
on the mountain, and has since been purchased by the Solar 
Observatory. The optical work on the 60-inch mirror, which 
was also acquired by the Solar Observatory, was resumed 
by Ritchey in the optical shop at Pasadena. He also 
designed the mounting for this telescope, and the work of 
constructing it was soon undertaken. 



CHAPTER XIII 

ASTRONOMICAL ADVANTAGES OP HIGH ALTITUDES 

The recognition of the advantages of making astronomical 
observations at liio^h altitudes sfoes back to the time of New- 
ton, who wrote as follows in his Opticks (third edition, p. 98) : 

If the Theory of making Telescopes could at length be fully 
brought into practice, yet there would be certain Bounds beyond 
which Telescopes could not perform. For the Air through which 
we look upon the Stars, is in a perpetual Tremor; as may be seen 
by the tremulous Motion of Shadows cast from high Towers, and by 
the twinkling of the fix'd stars. * * * The only remedy is a most 
serene and quiet Air, such as may perhaps be found on the tops of 
the highest Mountains above the grosser Clouds. 

It will be observed from these remarks that a clear and 
transparent sky is not the only need of the astronomer. In 
their passage through our atmosphere the rays which are 
united by a telescope to form the image of a star traverse 
different paths, depending upon their color. For air, like 
water or glass, though in a less degree, is a refracting medi- 
um; i. e., a ray of light entering it is bent from its straight 
course, and the amount of its bending depends upon the 
color of the ray, just as in the case of a prism. Violet light 
suffers the greatest refraction, and red light the least. Ob- 
viously, then, rays of different colors coming to a telescope 
from a star do not pursue the same path. Since the degree 
of refraction depends upon the temperature of the air, and 
since, under ordinary conditions, the temperature is chan- 
ging in an irregular manner, we thus see why a star twinkles 
and undergoes rapid change of color. For the red rays may 
be momentarily reduced in brightness, through a change in 

111 



112 Stellae Evolution 

refraction of the air through which they pass. The star 
would thus appear blue for the time being. The next 
instant the intensity of the blue light might be reduced, 
causing the star to seem red. Since the length of the light- 
path and the degree of refraction increase toward the hori- 
zon, the twinkling of stars, which frequently disappears alto- 
gether at the zenith, is most apparent at low altitudes. 

As the effect of twinkling is so apparent to the eye, it is 
easy to see that it may be greatly magnified in a telescope 
and produce serious interference with observations. The 
star image, instead of being a minute, sharply defined point, 
usually appears in the telescope enlarged, confused, and 
tremulous. The component members of close double stars, 
though easily within the resolving power of the telescope, 
under such conditions may overlap and appear as one. Simi- 
larly the minute surface details of the Moon or planets may 
be entirely obliterated by atmospheric disturbance. It is as 
though the astronomer were forced to observe the heavenly 
bodies from the bottom of an ocean, not calm and tranquil 
throughout its mass, but constantly disturbed by currents of 
various directions and at different depths, and by irregu- 
larities of density arising from unequal temperatures. 

It sometimes happens that excellent definition of tele- 
scopic images is obtained through smoke or haze, under cir- 
cumstances which might appear to be wholly unsuitable for 
astronomical work. For certain kinds of observations, where 
perfect definition is allimportant and brightness of the image 
of less consequence, the lack of transparency occasioned by 
hazy air does no harm. But in most classes of work particles 
suspended in the atmosphere not only reduce the inten- 
sity of the light, but produce serious interference through 
scattering of the rays. The brightness of the sky near the 
Sun, for example, increases greatly with the number of dust 
or smoke particles in the air. In visual observations of the 



Advantages of High Altitudes 113 

details of sun-spots this might not be harmful; but the visi- 
bility of the prominences is seriously reduced when they are 
seen against a brilliant background of sky. The brightness 
of stars is also much affected by haziness of the atmosphere. 

Even on a clear and transparent night the stars are less 
brilliant at sea level than when seen from the summit of a 
high mountain. For the air itself is a powerful absorbing 
medium and reduces, more than we ordinarily realize, the 
brightness of objects seen through it. Illustrations of the 
relative advantages of photographing stars at altitudes of 
1,200 and 6,000 feet respectively is given in Plates LVI 
and LYII. 

The difficulties in astronomical observations arising from 
atmospheric disturbances increase with the aperture of the 
telescope employed. This is because the rays falling on 
opposite sides of a large object-glass traverse more widely 
separated paths than those united by a small object-glass. 
They are thus liable to greater atmospheric disturbance, on 
account of the difference in the conditions governing the 
refraction of the light along the two paths. The disturbances 
of the air take the form of more or less regular waves. With 
an aperture which is small compared with the length of one 
of these waves, the effect on the image might not be great. 
If, however, several waves were included within the aperture, 
the confusion might be very marked indeed. Hence large 
telescopes require better conditions than small ones. 

In selecting the site of the Yerkes Observatory, practical 
considerations necessarily limited the choice. It was essential 
that the observatory should be situated within easy reach of 
the university, and this fact rendered it impossible to consider 
seriously the favorable mountain regions which were known 
to exist in the extreme western part of the United States. 
The chosen site has many advantages over points in the 
immediate neighborhood of Chicago. The absence of smoke 



114 Stellar Evolution 

and the brilliant illumination of the sky produced in large 
cities by electric lights, the freedom from vibration arising 
from railways and the heavy traffic of a large city, and the 
facilities for quiet study afforded by the tranquil life of the 
country, were important recommendations of the Lake 
Geneva site. The observational work of the Yerkes Obser- 
vatory has been sufficient in amount and quality to show 
that more valuable material can be secured under such 
atmospheric conditions than can be adequately discussed 
without a far larger staff of computers than the observatory 
has ever been able to employ. It goes without saying, how- 
ever, that a better site would have been preferable. 

But it must not be supposed, from what has been said, 
that all mountain peaks would make good observing stations. 
It is true that by ascending into the upper atmosphere the 
astronomer may escape the strong absorption exercised by 
the dense air of lower levels. As one goes up, the stars 
become brighter and brighter, especially near the horizon, 
since the decrease in length of path is much greater in this 
region than near the zenith. Blue and violet light suffer 
more from atmospheric absorption than the red, yellow, and 
green rays. For this reason, the advantages of high eleva- 
tions, so far as transparency is concerned, are more apparent 
in photographic than in visual observations, since the blue 
and violet rays are principally concerned in the production 
of the photographic image. 

Thus, from the standpoint of atmospheric transparency, 
mountain sites may always be considered to possess advan- 
tages for astronomical work. But transparency is almost 
invariably a much less important consideration than sharp- 
ness of definition, which does not, by any means, depend 
merely upon altitude. In the first place, the geographic 
location of the mountain in question is a most important 
factor. Long periods of continuous clear weather, enjoyed 



Advantages of High Altitudes 115 

in certain favored regions, are accompanied by a uniformity 
of atmospheric conditions unknown in countries where storms 
usually prevail. It is not merely that clouds and rain are 
less common; for, if this were the only important considera- 
tion, a clear night in one part of the world might be as good 
for astronomical purposes as an equally clear night in an- 
other. In a region of storms the disturbances follow one 
another so rapidly that during the intervening periods of 
clear weather the atmosphere rarely has time to settle down 
to a calm, homogeneous state. In southern California, for 
example, the sky is almost constantly clear for many months 
in the year, and the uniformity of the atmosphere is shown 
by the steadiness of the barometer and the low average wind 
velocity. During the rainy season, however, when storms 
may recur in rapid succession, the atmosphere in such a 
region is disturbed, and the conditions for astronomical work 
on the beautifully transparent nights that intervene between 
storms are frequently no better than in the eastern part of 
the United States. 

Pike's Peak (14.147 feet) affords an example of a moun- 
tain site poorly adapted for astronomical purposes. In June 
and July of 1893 I spent two weeks there, in company 
with Keeler, engaged in an attempt to photograph the solar 
corona without an eclipse. Under normal conditions the 
sky, as seen from the peak, is of a deep blue by day, and 
very transparent by night. The conditions, therefore, are 
favorable for work in which transparency is the only important 
consideration. Thus Pike's Peak might serve very well for 
the measurement of the solar radiation, were it not for the 
fact that during the summer months (always the most 
important season for solar work), the mountain is frequently 
capped by clouds through a considerable part of the day. 
On many of the nights during our stay the sky was perfectly 
clear, and remained so until about nine o'clock in the morn- 



116 Stellar Evolution 

ing. Then small cumulus clouds would begin to form imme- 
diately around the peak, and by noon a thunderstorm would 
be raging, frequently accompanied by a light fall of snow. 
In these storms the wind rose to a tremendous velocity, some- 
times as great as seventy miles an hour, and the electrical 
phenomena were very remarkable. The frequency with which 
these storms cut off all solar observations, except in the early 
morning, illustrates the fact that even for work on the solar 
radiation, which requires a clear and transparent sky through 
the greater part of the day, Pike's Peak would serve but 
poorly, at least daring this season of the year. As many of 
these storms were confined to the immediate summit of the 
mountain, a station several thousand feet below would prob- 
ably offer more opportunities for work than the peak itself. 

But this is not all. The definition of the Sun or stars is 
rarely good on Pike's Peak. This is probably due, not merely 
to frequent storms and high wind velocities, but also in part 
to the fact that the summit of the mountain is bare and rocky, 
so that heated currents of air rise from the surface and ruin 
the definition of the solar image. At this altitude mountain 
sickness is also very common, and would undoubtedly inter- 
fere, in some degree, with the operation of an observatory. 
The observers at that time stationed there by the Weather 
Bureau informed us that they could not remain on the 
mountain for long periods without impairment of health and 
energy. Two-thirds of the tourists who came to the summit, 
by the railway or on foot, were visibly affected by the high 
altitude. Another cause of difficulty at the time was forest 
fires in the mountains surrounding the peak, which sent 
volumes of smoke into the air. This rose to a great altitude 
and destroyed the deep blue of the sky. 

The unsuccessful attempts to photograph the corona were 
renewed on Mount Etna in July, 1894, through the kindness 
of Professor Ricco, director of the Bellini Observatories 



Advantages of High Altitudes 117 

of Catania and Mount Etna. Our party, consisting of 
Professor Ricco, Signorina Ricco, Antonino Capra, mecha- 
nician of the observatories, Mrs. Hale, and myself, left 
Catania on July 7. After a drive of three hours we arrived 
at Nicolosi, where we spent the night. The following extracts 
from my diary relate mainly to the atmospheric conditions 
encountered : 

July 8. Left Nicolosi at 6 a. m. Arrived at Casa del Bosco 
(4,760 feet) at 8^ 30"^. Examined sky frequently, and found slight 
decrease of white as we ascended. Crossed lava stream of 1892, 
and had excellent view of the craters of that year, the latest of which 
still emits vapor. Arrived at the observatory (9,650 feet) at 1^ 35°i. 
The temperature had fallen to 9" C, and the sky was nearly covered 
with clouds. Half an hour later we were enveloped in cloud, which 
surrounded us until evening, when sky was whitish, with marked 
halo around Moon. Stars unsteady, even in zenith. 

July 9. Sky clear, with strong wind blowing the smoke from 
the great crater (which rose behind the observatory to an altitude 
of 10,900 feet) away from the direction of the Sun. Half the island 
of Sicily was dimly visible from the observatory through a great 
brown bank of thick haze, the upper surface of which seemed to 
be nearly on a level with us. Cumulus clouds commenced to form 
at 9^, and soon the sky was nearly covered. At 12^ the Sun was seen 
between passing clouds to be surrounded by a bright halo. Wind 
changed to west in the afternoon, and sky became much whiter. 

July 10. Wind blew smoke of great crater over Sun, making 
sky very white. Observed Sun with Professor Ricco by projection 
with 12-inch telescope. Image rather better than at Catania, but 
became unsteady later. At 10^ some small cumulus clouds had 
formed, and Sun was surrounded by bright halo. Clouds of insects 
were also noticed in direction of Sun, as on Pike's Peak. Observed 
prominences with Professor Ricco, but images were no better than 
at Catania. At sunset watched shadow of Etna from the Torre del 
Filosofo. Whole sky covered with dense haze. 

July 11. Sky very white, bright ring around Sun. Observed 
atmospheric lines with direct-vision spectroscope. Balanced tele- 
scope, and observed Sun by projection. Seeing excellent; granu- 
lation, spots, and f aculae well defined. Strong odor of sulphur. At 



118 Stellar Evolution 

sunset visited Valle del Bove. Sky filled with haze, and almost too 
bright for the eye 10° from Sun. 

July 12. Sky very white. Wind still blowing smoke from crater 
over Sun. Bank of haze above level of observatory. Observed Sun 
by projection with Professor Ricco; image unsteady. Climbed to 
top of crater, and found sky in zenith of deeper blue than when seen 
from observatory. Whole island enveloped in haze. Descended 
to observatory by moonlight; double halo around Moon. Observed 
Moon, Saturn, and several stars with the 12-inch, using powers up 
to 430. Seeing magnificent; images almost perfectly steady with 
highest power. Both Moon and Saturn were very low, but images 
were remarkably good. With naked eye scintillation was hardly 
perceptible in stars higher than 30°. 

July 13. Wind blowing from direction of crater, but sky best 
since July 9: cloudless and generally whitish, but increase in bright- 
ness toward Sun was gradual. Much dust. Telescope in use until 
gh 40™ by Professor Ricco for daily record of chromosphere. Prom- 
inences very well seen. At 9^^ 50"fi broad and brilliant ring of 
w^hiteness around Sun, making it useless to try for corona. Smoke 
blowing directly over Sun, and diffusing through entire sky. Solar 
image observed by projection; definition very poor. At 11 ^ sky had 
improved, and preparations were made to photograph corona, but 
five minutes later more smoke blew over Sun, and sky became very 
white. Mirror found to be dewed, and surface badly tarnished by 
the sulphurous fumes, though it had been tightly covered every 
moment it was not in use. Sky around Sun remained bright, and 
wind was so violent that no photographs could be made. Strong 
sulphurous odor. 

July 14. Smoke blowing across sun. Strong sulphurous odor. 
Whole eastern sky white. Prominences fairly well seen at T^^ 45™. 
Left observatory at 3 h, and arrived at Catania about midnight. 

As I was assured by Tacchini and Ricco that the sky is 
frequently very clear on Etna, it may safely be concluded 
that the difficulties we encountered w^ere exceptional. During 
the entire time of our stay in southern Italy and Sicily the 
atmosphere was very hazy, and the sky was rarely of a deep 
blue. I was told by Galvagno, the custodian of the Etna 
Observatory, that the smoke this year was much more notice- 



Advantages of High Altitudes 119 

able than usual. If the wind had blown it away from, instead 
of toward, us, the sky would probably have been pure, though 
hardly as blue as when seen from Pike's Peak during the 
first part of our visit there. ^ 

So much for the results of brief personal experience in 
Sicily and the Rocky Mountains. From the standpoint of 
a solar observer requiring fine definition, they do not appear 
very encouraging. Moreover, conclusions reached by other 
astronomers have been equally unfavorable to Colorado air; 
and we find Piazzi Smith, in his book Teneriffe: An Asfroii- 
omer^s Experiment, reporting but very little good solar 
definition at altitudes up to 10,700 feet on a tropical island. 
His expedition to Teneriffe in 1856, made for the express 
purpose of testing the atmospheric conditions on a mountain- 
peak, was the first serious study of this kind. The trans- 
parency of the air and the definition of the stars by night 
were found to be excellent; but high winds, dust in the 
upper atmosphere, and unsteady solar images were also 
encountered. 

However, good solar definition is experienced on Mont 
Blanc (15,780 feet), at the Kodaikanal Solar Observatory 
in India (7,700 feet), and at the Pic-du-Midi in France. 
There is obviously no incompatibility between high altitudes 
and good solar definition. The poor definition reported by 
various observers on mountain-peaks is due either to the 
prevalence of storms or to local disturbances, caused by 
warm air rising from the heated summits of mountain-tops 
protected by little or no foliage. At Mount Hamilton, where 
the night conditions are so favorable, the slopes immediately 
around the summit are composed of bare rock, which becomes 
intensely heated and necessarily affects the solar definition. 
This is a matter of no special consequence to the Lick 

iThe attempts to photograph the corona were continued by Ricc6 under better 
conditions, but neither this method nor any other has yet proved successful. 



120 Stellak Evolution 

Observatory (Plate XL VIII), since the work is confined to 
night observations. The great number of admirable results, 
many of them requiring the finest definition, which have 
been obtained at the Lick Observatory, afford the best of 
evidence that its site was well chosen. 

The results of experience in various parts of the world 
would seem to indicate that a mountain observatory, if it is 
to enjoy good conditions both by night and by day, should 
be situated in a climate where the sky is clear continuously 
for periods of several weeks or months, and the average wind 
velocity is low. The summit of the mountain, as well as its 
slopes, should be covered with foliage, to protect it from the 
heat of the Sun. Finally, the elevation should be sufficient 
to escape the dust which diffuses itself through the air in 
the dry season, and the low-lying fogs and clouds frequently 
encountered in regions near the sea. 



CHAPTER XIV 
THE MOUNT WILSON SOLAR OBSERVATORY 

From the preceding chapters, it will be seen how the plan 
of research of the Solar Observatory was developed. At 
Kenwood a programme of solar observations, involving the 
use of the spectroheliograph, the photographic study of the 
spectra of Sun-spots and other solar phenomena, and the fullest 
possible application of laboratory methods in astrophysical 
research, was instituted. At the Yerkes Observatory this 
programme was broadened and extended, in the hope of 
providing ultimately for the general study of stellar evolu- 
tion; the possibilities of the spectroheliograph were more 
fully realized, through the advantages offered by the 40-inch 
refractor; and instruments better adapted than the large 
refractor for the further prosecution of the work, such as the 
Snow telescope for solar research, and the 60-inch reflector 
for stellar investigations, were designed and partially or 
wholly constructed. After this period of preparation, devoted 
in large part to the development of plans and methods, the 
Mount Wilson Solar Observatory was organized for the study 
of stellar evolution, at a station enjoying the best climatic 
advantages. 

In brief, the scheme of research of the Solar Observatory 
comprises: (1) solar investigations, to contribute toward our 
knowledge of the Sun («) as a typical star and (5) as the 
central body of the solar system; (2) photographic and 
spectroscopic studies of stars and nebulae, bearing directly 
upon the physical nature of these bodies, with special refer- 
ence to their development; (3) laboratory investigations, for 
the interpretation of solar and stellar phenomena. With 

121 



122 Stellae Evolution 

the central problem in mind, each successive research is 
designed to occupy a logical place in a concentrated attack, 
proceeding along these converging lines. 

The variety of the problems connected with the establish- 
ment of the Solar Observatory on Mount Wilson affords a 
good illustration of the diversified work of an astronomer. 
It was necessary, in the first place, to test the atmospheric 
conditions by means of telescopic and meteorological observa- 
tions extending over a considerable period of time, in order 
to make certain that the site would prove suitable. In the 
second place, since the summit could be reached only by a 
narrow mountain trail, it was evident from the outset that 
the question of transporting building materials and the parts 
of heavy instruments would not be an easy one to solve. 
Again, since one of the prime purposes of the new observatory 
was to take advantage of the possibilities of improved instru- 
ments, the design and construction of the telescopes, spectro- 
scopes, and other appliances would require the solution of 
many instrumental and engineering problems, and much work 
of experiment. It was known, for example, that glass mirrors 
change their form decidedly when exposed to the Sun's rays. 
For this reason it was to be feared that they might not give 
good solar images. This is a matter of fundamental impor- 
tance, since the fixed telescope for solar observations neces- 
sarily involves the employment of mirrors. In addition to 
these questions, many others, very diverse in character, pre- 
sented themselves. These included the preparation of a 
programme of research, adapted for the special requirements 
of the new observatory, in which all the investigations in 
progress were to be closely correlated; the consideration of 
the best methods of discussing and interpreting the photo- 
graphs made with the spectroheliograph and other instru- 
ments; the invention and construction of special measuring 
and computing machines, etc. 



Mount Wilson Solae Observatory 123 

From a meteorological standpoint, the state of California 
may be divided into three parts. In the northern region 
the rainfall is very considerable, much clondiness prevails, 
and in almost all respects the conditions are unfavorable for 
astronomical work. The central region, which may be con- 
sidered to extend as far south as Point Conception, is favored 
with much better weather conditions, best exemplified at the 
Lick Observatory, on Mount Hamilton, where a high average 
of night-seeing is maintained during a large part of the year. 
In the southern part of California the climatic conditions are 
different from those which prevail in the two other sections 
of the state. The lighter rainfall is naturally associated with 
fewer clouds, a remarkably steady barometer, and very light 
winds. 

There can be no doubt that the character of the country 
immediately adjoining an observatory site affects the condi- 
tions for astronomical work to an important degree. For this 
reason it became desirable to make preliminary tests of a con- 
siderable number of points in southern California. Similar 
tests might have been desirable in Arizona, were it not for 
the thunderstorms that prevail during the summer months in 
the vicinity of Flagstaff, and other promising localities, which 
would interfere so seriously with solar work as to put this 
region almost entirely out of consideration. As there were 
other serious objections to Arizona sites, and as Hussey's tests 
at Flagstaff did not indicate that the conditions were as favor- 
able as in California, attention was concentrated on the rela- 
tive claims of various mountains in southern California. 

Hussey's tests in this region included Echo Mountain, 
Mount Lowe, and Mount Wilson, in the Sierra Madre range, 
and Cuyamaca and Palomar, much farther to the south. 
His observations seemed to leave no doubt that Mount Wilson 
would prove to be the best site for the purposes of a solar 
observatory. 



124 Stellae Evolution 

Mount Wilson is one of many mountains that form the 
southern boundary of the Sierra Madre range (Plate XLIX). 
Standing at a distance of thirty miles from the ocean, it rises 
abruptly from the valley floor, flanked only by a few spurs of 
lesser elevation, of which Mount Harvard is the highest. 
Except for a narrow saddle. Mount Wilson is separated from 
Mount Harvard by a deep canon, the walls of which are very 
precipitous. Farther to the west, beyond the saddle leading 
to Mount Harvard, the ridge of Mount Wilson forms the upper 
extremity of Eaton Canon, which leads directly to the San 
Gabriel Valley. East and north of Mount Wilson lies the 
deep canon through which flows the west fork of the San 
Gabriel River, and beyond this rise a constant succession of 
mountains, most of them higher than Mount Wilson, which 
extend in a broken mass to the Mojave Desert. The Sierra 
Madre range forms the northern boundary of the San 
Gabriel Valley, which is further protected toward the east 
from the desert by the high peaks of the San Bernardino 
range. 

The view from the summit of Mount Wilson is most exten- 
sive, embracing the whole of southern California, and reach- 
ing out over the Pacific to islands nearly one hundred miles 
distant. Cuyamaca, about 130 miles to the south, not far 
from the Mexican boundary, is easily visible. San Bernardino 
and San Jacinto peaks, the latter 90 miles away, are so dis- 
tinctly seen under normal conditions that a station, might 
easily be established on either of them, for experiments in 
measuring the velocity of light from Mount Wilson. Mount 
San Antonio (10,080 feet), 25 miles away, has already served 
as a station for certain observations of the solar radiation, 
supplementing the work of the Smithsonian Expedition at 
Mount Wilson (Plate L). 

During a part of the year, particularly from April to 
August, fog rolls in from the ocean and covers much of the 



Mount Wilson Solar Observatory 125 

San Gabriel Valley during the night ( Plate LI). But these 
fog-clouds rarely attain elevations exceeding 3,000 feet. The 
mountains of the Sierra Madre range rise high above the 
fog, and during many months of the year they enjoy practi- 
cally continuous sunshine. In summer the sea breeze blows 
for a part of the day, but it attains only a low velocity, 
which decreases in passing from the valley to the moun- 
tain tops. 

Mount Wilson is reached from the San Gabriel Valley by 
either one of two trails. One of these, known as the "Wilson 
Trail," ascends from Sierra Madre, and is steep and irregular. 
The other, called the "New Trail," rises from the foot of 
Eaton Canon, about 6|^ miles from Pasadena, and is about 
9^ miles long. When our work commenced, it was but little 
over two feet in width at its narrowest parts. It has an average 
grade of about 10 per cent., and is much better adapted for 
transportation purposes than the old Wilson Trail. 

Some hundreds of tons of building material for the 
observatory have been taken over the New Trail, on the backs 
of mules or "burros" (donkeys) (Plate LII). The heavier 
parts of instruments, which could not be taken up in this 
way, were carried on a special truck built for the purpose 
(Plate LIII). The running-gear consists of four automobile 
wheels with rubber tires. The body of the truck is hung by 
wrought-iron yokes from the running-gear, with its lower 
surface at a height of only six inches above the ground. 
Steering-gear, of the type used on automobiles, is provided 
for both pairs of wheels. A man riding on the load steers 
the forward wheels, while the rear wheels are steered with a 
tiller by a man walking behind the carriage. A single large 
horse pulls a load of a thousand pounds on this carriage with- 
out difficulty. With two horses, used in relays, the trip from 
the lower end of the trail to the summit and return is com- 
pleted with such a load in about fifteen hours. About sixty 



126 Stellar Evolution 

round trips were made with this truck for the purpose of 
carrying the mirrors, lenses, and heavy castings of the Snow 
and Bruce telescopes, the parts of a 15-H. P. gas engine, 
and other heavy machines, as well as the 4-inch pipe columns 
used in constructing the steel skeleton of the Snow telescope 
house. 

During the first two years, it was hoped that a railway 
would be constructed to the summit of the mountain, where 
a hotel had already been erected. When it finally appeared 
that this hope must be abandoned, we were compelled to 
adopt the alternative of widening the New Trail into a wagon- 
road (Plate LIV). This work, which was done during the 
autumn and spring of 1906 and 1907, was considerably ham- 
pered by unprecedented storms in December and January. 
The snow on the summit of Mount Wilson (Plate LV) was 
five feet deep on a level, and the torrential rains, below the 
snow line, brought down thousands of tons of earth and rocks 
from the steep slopes of the mountain. When these difficul- 
ties had been overcome, the transportation problem was so 
far solved as to permit the structural steel for the building 
and dome of the 60-inch reflector to be hauled to their 
destination. 

Our systematic tests of the atmospheric conditions on 
Mount Wilson began in March, 1904. An old log cabin, 
which had been in a state of partial ruin, was rendered 
habitable and occupied until the "Monastery" was com- 
pleted, in the following December. Frequent tests of the 
solar definition were made with a 3J-inch refracting tele- 
scope, supplemented by meteorological observations. 

The specific requirements of a site for an observatory to 
be devoted to solar research and the study of stellar evolu- 
tion are as follows: 

1. Excellent definition of the solar image, on many days 
of the year. 



Mount Wilson Solae Obsekvatory 127 

2. Excellent definition by night, so as to permit reflecting 
telescopes of large aperture to be used for the most exacting 
work. 

3. Great transparency of the day and night sky, essential 
for accurate determinations of the "solar constant" (the 
total heat radiation of the Sun, at a point outside of the 
Earth's atmosphere), and the photography of stars and 
nebulae requiring very long exposures. 

4z. Continuous clear weather for periods of many weeks, 
rendering possible daily observations of changing phenom- 
ena, of which an imperfect or erroneous idea might be derived 
from scattered observations. 

5. A low average wind velocity, especially during the 
best observing season, to insure freedom from vibration of 
telescopes employed for photographic work. 

It is easy to see why the definition of the Sun's image is 
usually much inferior to that of the stars or planets. The 
heating of the earth, caused by the Sun's rays, produces 
currents of warm air, which rise and mix with the cooler air 
above. It has already been explained that poor definition is 
produced by irregular refraction in the atmosphere, and that 
this is caused by irregularities in the temperature of the air 
through which the light rays pass. In this respect a moun- 
tain peak may have some disadvantages as compared with an 
extensive level area, because the rising currents of warm air 
follow the mountain sides and tend to produce marked dis- 
turbances in the images observed from the summit. It is 
evident that this effect will be greatly enhanced if the moun- 
tain is bare and rocky, instead of having its slopes covered 
with trees and bushes. As the latter condition prevails on 
most of the slopes of Mount Wilson, the heating of the air 
is much less pronounced than in the case of many other 
mountains. It is nevertheless very noticeable, and for this 
reason the best observations of the Sun are made one or two 



128 Stellar Evolution 

hours after sunrise, and about the same time before sunset. 
It is true that great depths of atmosphere must be traversed 
by the Sun's rays when it is so near the horizon. Neverthe- 
less, the image on a large number of days in the summer 
season is wonderfully sharp and distinct, permitting the 
finest details of structure to be "observed. The conclusions 
based upon observations made with the 3J-inch refractor 
were afterward confirmed with the large aperture of the 
Snow telescope, leaving no doubt that with respect to solar 
definition Mount Wilson offers very exceptional advantages. 
The tests of the night definition, and of the transparency 
of the night sky, were made by Barnard, daring the work 
of the Hooker Expedition. In chap, v a description has 
been given of the Bruce 10-inch photographic telescope 
of the Yerkes Observatory, used by Barnard in his studies 
of the Milky Way. In order to extend farther south the 
work previously done with an instrument of 6 inches aper- 
ture on Mount Hamilton, Barnard brought the Bruce tele- 
scope to Mount Wilson and made with it a remarkable series 
of photographs. Mount Wilson (latitude + 31° 13') is 8° 
south of the Yerkes Observatory, and 3° south of the Lick Ob- 
servatory. This fact, combined with the great transparency 
of the sky, permitted Barnard to photograph regions of the 
Milky Way which had been out of reach in his earlier work. 
» The best way of comparing the transparency of the sky 
at Lake Geneva and Mount Wilson is by taking two photo- 
graphs of the same region of the heavens, with the same 
exposure time, on photographic plates of the same sensitive- 
ness, used with the same telescope, by the same observer. 
Such a comparison is illustrated in Plate LVI, which repre- 
sents the cluster Messier 35. The difference in the number of 
stars included on the photograph is a striking illustration of 
the advantages of Mount Wilson. Indeed, if this result were 
not confirmed by many others, and regarded by Barnard 



Mount Wilson Solar Observatory 129 

as representing a fair relative test, it might be supposed 
that some difference in the mode of development or in the 
sensitiveness of the plate had entered. The night on which 
the Mount Wilson photograph was made was an average sum- 
mer night, while in the case of the Yerkes Observatory 
photograph the transparency was possibly higher than the 
average there. 

An illustration of the same sort is given in Plate LVII, 
which shows the Pleiades as photographed by Barnard with 
the Bruce telescope, with an exposure of 3 hours and 48 
minutes at Mount Wilson and 9 hours and 47 minutes at 
the Yerkes Observatory. It will be seen that the first photo- 
graph shows quite as many stars as the second, and also 
has a great advantage in sharpness, as indicated by the 
much larger amount of detail brought out in the nebulae. 
This is due to the fact that the greater diffusion of light in 
the Wisconsin sky tends to obliterate the finer details of the 
photograph. It is interesting to conjecture what advantages 
will result from the use of the 60-inch reflector under these 
fine conditions. 

During the long exposures Barnard kept a star on a pair 
of cross-hairs in the eye-piece of a 5-inch refractor, attached 
to the Bruce telescope. In this way he observed the defini- 
tion of the stellar images on a large number of nights. As 
previously explained, the definition of a star does not depend 
in any considerable degree upon the transparency of the 
atmosphere, but rather upon the absence of irregular refrac- 
tion. Barnard found the average night "seeing" to be 
remarkably good, and this conclusion has also been con- 
firmed with the large aperture of the Snow telescope. 

The transparency of the sky by day has been most 
thoroughly tested by Abbot, in his studies of the "solar 
constant" of radiation, which are described in chap. xxii. 
As compared with Washington, where the previous work 



130 Stellar Evolution 

of the Smithsonian Astrophysical Observatory has been 
done, the advantages of Mount Wilson are very marked. 
Of equal importance for this work is the fact that the 
observations can be made day after day, with practically no 
interruption, for a period of many weeks. In Washington, 
during the same period, it might be possible to obtain only 
two or three trustworthy determinations. Thus the manner 
in which the solar radiation varies can be shown, in the one 
case, by its daily fluctuations, while in the other it might be 
wholly concealed. 

Finally, the average wind velocity in the dry season 
proved to be extraordinarily low, not only for an exposed 
mountain-peak, but as compared with a station at any level. 
During the rainy season, when there is much cloudy weather, 
violent storms, accompanied by high winds, are not uncom- 
mon. But in the dry season an almost dead calm frequently 
prevails at night, and also during the early morning solar 
observations. In the later hours of the day there is usually 
a light breeze. The typical condition on Mount Wilson 
during the dry season may be described as a perfectly cloud- 
less sky, and so little breeze that the leaves are hardly 
stirred by it. 

It would be tedious to discuss the other conditions, such 
as the heavy growth of foliage, the presence of abundant 
springs of water, the neighborhood of large cities, etc., which 
contribute toward the advantages of Mount Wilson as an 
observatory site. The astronomical tests have been described 
in detail because they illustrate the practical bearing of 
atmospheric conditions on astronomical observations. 



CHAPTER XV 

THE SNOW TELESCOPE 

Leon Foucault appears to have been the first to appre- 
ciate the advantages of a fixed telescope, capable of forming 
a solar or stellar image within a laboratory. A large sidero- 
stat, constructed by Eichens after his designs, was completed 
in 1868, the year of Foucault's death. It remained at the 
Paris Observatory, where it was subsequently employed by 
Deslandres for solar photography. For small images of the 
Sun this instrument gave good results, although the imper- 
fection of the driving caused the image to wander more or 
less from a fixed position. This difficulty has been inherent 
in almost all types of fixed telescopes. It is coupled with 
the inconvenience that the solar image produced by the 
siderostat or heliostat rotates in an irregular manner, which 
would, cause distortion in long-exposure photography with a 
fixed spectroheliograph. 

It is not easy to see why the heliostat, in some of its 
forms, was not more rapidly developed. With a few excep- 
tions, its practical application has been confined to small 
heliostats of various types, used to reflect a beam of sunlight 
into the laboratory, but not to produce a large image of the 
Sun. In other words, the heliostat was not developed into 
an instrument of precision, capable of giving a large and 
well-defined solar image, and maintaining it accurately fixed 
in position, until the coelostat was revived for eclipse pur- 
poses, about ten years ago.^ This instrument had been 
invented long before, but its great advantages, due to the 

iThe great equatorial coude of the Paris Observatory is an admirable example 
of a fixed telescope, but I do not think it has been tested for solar observations. 

131 



132 Stellae Evolution 

simplicity of its construction, the ease of driving it with a 
precision as great as in the case of the equatorial refractor, 
and, above all, the fact that the solar image produced by it 
does not rotate, had been overlooked. At Turner's sugges- 
tion it was employed for eclipse purposes, at first by some 
of the English parties, and subsequently by astronomers in 
all parts of the world. 

However, the conditions under which eclipse observations 
are made are very different from those that obtain in ordi- 
nary solar work. A defect of the coelostat is that the direc- 
tion of the beam of light reflected horizontally from the 
mirror varies with the declination of the Sun. During the 
few minutes of a total eclipse the Sun's declination does not 
change appreciably, and the telescope into which the light 
is reflected by the coelostat stands fixed in position. But 
in solar observations continued throughout the year the 
direction of the reflected beam is constantly changing, as 
the Sun moves north or south of the equator. As it would 
be inconvenient to swing the long telescope tube, which is 
pointed at the coelostat, around through the necessarily large 
angle, a second mirror must be introduced to receive the 
light reflected from the coelostat mirror and send it in any 
desired direction. About once a week, or sufficiently often 
to cause no appreciable loss of sunlight, the second mirror 
is moved a short distance, so that it may continue to receive 
all the light of the reflected beam, in the changed position 
given it by the variation in the Sun's declination. 

Another difficulty of the coelostat, which is common to 
all forms of heliostat, but plays no part in total-eclipse work, 
is the distortion of the mirrors by sunlight. This obstacle 
is really the only serious one presented by this form of 
telescope. How it has been met at the Solar Observatory 
is explained below. 

The Snow telescope, the optical and mechanical parts of 



The Snow Telescope 



133 



which were constructed under Hitch ey's supervision in the 
shops of the Yerkes Observatory, is illustrated in Plate 
LVIII. This photograph shows the coelostat and the adjust- 
able second mirror, whence the light is reflected to a concave 
mirror of 60 feet focal length, which forms the solar image. 
The general arrangement of the telescope, as established on 
Mount Wilson, is indicated in Fig. 5. The coelostat stands 




FIG. 5 
Plan and Elevation of Snow Telescope House on Mount Wilson 



on a carriage, which can be moved east or west along the 
line act. On account of the configuration of the ground, 
which falls rapidly toward the north, it was necessary to 
make the long axis of the building run 15° east of north, 
instead of being exactly in the meridian. For the same 
reason this axis is not horizontal, but inclined downward 5° 
toward the north. Without these adaptations of the plan to 
the conditions of the site, the height of the northern part of 
the building would have been very great, involving serious 
increase of expense. The rails 66, on which the carriage 
bearing the second mirror slides, are parallel to the optical 
axis. The coelostat mirror, 30 inches in diameter, and the 
second mirror, 24 inches in diameter, have plane surfaces, 



134 Stellae Evolution 

and serve merely for bringing the sunlight into the telescope 
house. The plane of the coelostat mirror is parallel to the 
Earth's axis, and the mirror can be rotated around this axis 
once in forty-eight hours, by means of a driving-clock. This 
exactly counteracts the motion of the beam due to the Sun's 
apparent motion through the heavens. 

From the second mirror the light passes to either one of 
two concave mirrors, each 24 inches in diameter (Plate LIX) . 
One of these, which has a focal length of 60 feet, is supported 
on a carriage so that it can be moved (for focusing) along 
the rails cc, which are mounted on the small pier shown near 
the middle of Fig. 5. This mirror produces an image 
of the Sun about 6.7 inches in diameter, at a position in the 
spectroscope house determined by the angle which the con- 
cave mirror makes with the incident beam of sunlight. If 
the mirror stood normal to the beam, the sunlight would be 
reflected directly back upon itself toward the second mirror. 
If, however, the concave mirror is turned slightly to one side, 
the solar image can be formed at the end of the pier /, where 
the 5-foot spectroheliograph stands. By moving the mirror 
back toward the north, along the rails on which it slides, the 
image can be brought to a focus on the pier i, where the slit 
and photographic plate of a Littrow spectrograph, of 18 feet 
focal length, are mounted. Again, by rotating the concave 
mirror so as to return the beam in a somewhat different direc- 
tion, the solar image can be sent into the constant tempera- 
ture room III, where the bolographic apparatus, for studying 
the heat radiation of different parts of the Sun, is mounted 
on the massive triangular pier kkk. 

If a larger solar image is required, the mirror of 60 feet 
focal length is moved out of the way , and the beam from the 
second mirror allowed to pass to a concave mirror of 143 
feet focal length, mounted on a pier at the extreme north 
end of the telescope house. The image of the Sun is then 



The Snow Telescope 135 

formed at the pier g, 143 feet from the concave mirror. 
This image is 16 inches in diameter, and is used for the 
special study of solar details, for which a large scale is 
required. 

The remarkable convenience of such a telescope, when 
contrasted with a great movable refractor like the Yerkes 
telescope, is immediately evident. Instead of attaching each 
heavy instrument, one by one, to the end of a moving tele- 
scope tube, it is set up once for all on a pier, where its 
adjustments need never be disturbed. It is thus possible to 
pass rapidly from one instrument to another, photographing 
the forms of the calcium flocculi, for example, with the 
spectroheliograph, and their spectra, only a moment later, 
with the powerful Littrow spectrograph. In view of the 
importance of studying solar phenomena nearly simulta- 
neously by various methods, and of closely correlating the 
observations, the advantages afforded by such a telescope 
will be easily recognized. 

The peculiar form of house in which the Snow telescope 
is mounted calls for a word of explanation. In previous 
experiments, some of which were made on Mount Wilson in 
the spring of 1904, the conclusion was reached that the dis- 
turbance of the definition caused by warm air rising from 
the ground in the immediate neighborhood of the heliostat 
could be appreciably reduced by mounting the instrument 
at a considerable height. Observations made with a tele- 
scope supported in a tree, at various heights up to seventy 
feet, seemed to leave no doubt regarding this point. A 
second consideration, the importance of which had been 
particularly emphasized by experience with a smaller coelo- 
stat telescope, having a closed tube not provided with means 
of ventilation, was the necessity of designing a house so that 
the temperature within would be at all times as nearly as 
possible the same as that of the outer air. It is evident that, 



136 Stellar Evolution 

if this condition is not met, the mixture of air of different 
temperatures at the open end of the house, through which 
the beam enters, will cause irregular refraction and conse- 
quent disturbance of the image. 

Plate LX shows the pier on which the coelostat is 
mounted, at a height of nearly 30 feet above the ground. 
Since the parallel rays from the coelostat to the concave 
mirror pass through a closed house, it is not essential that that 
part of the building should stand high above the ground. It 
is important, however, that disturbances due to heating of 
the walls, caused by sunlight falling upon them, be obviated. 
For this reason all parts of the building, including the 
movable shelter, the spectroscopic laboratory, and the long 
narrow house extending north from the laboratory, have an 
inner wall and ceiling of canvas, and an outer wall composed 
of canvas louvers, very completely ventilated. The roof is 
also ventilated, by wooden louvers at the ridge throughout 
the entire length of the movable shelter and the north exten- 
sion, and at the peak of the laboratory. Rain and snow are 
prevented from entering the roof louvers by means of canvas 
guards, which can be raised or lowered at will. The house 
extending north from the laboratory has a floor of canvas, 
with a space below, through which the air may pass freely. 

The louvers surrounding the coelostat pier are intended 
to protect the pier from vibration caused by the wind, and 
from heating by the Sun. The steel structure does not touch 
the pier at any point, and is therefore made rigid enough to 
support itself in high winds. When not in use, the coelostat 
and second mirror are covered by a house on wheels, closed 
at both ends by walls of heavy canvas. These may be opened, 
so that when the house is moved to the north the coelostat 
stands fully exposed. The movable shelter then fits closely 
against the south wall of the laboratory, and forms a part of 
the tube through which the beam passes. 



The Snow Telescope 137 

In the preliminary tests of the Snow telescope at the 
Yerkes Observatory the results were rather disappointing, 
though good images were sometimes obtained. There was 
evidence of distortion of the mirrors by the Sun's heat, and 
in the first experiments on Mount Wilson similar difficulty 
was experienced. Soon after the exposure of the mirrors to 
the Sun it was seen that the focal length was increasing, and, 
as the focus changed, evidence of astigmatism, due to the 
distortion of the plane mirrors, made itself apparent in the 
appearance of the image inside and outside of the focal 
plane. It was soon found that the focus changed much more 
rapidly after the mirrors had been silvered for some time, 
because of the greater absorption of heat by the slightly tar- 
nished surfaces. Moreover, the change was less on a day 
with a cool breeze than on a day with no wind. The question 
then arose whether this difficulty could be remedied. 

In the early morning, when, as before stated, the defini- 
tion of the Sun is best, the heating is much less marked than 
later in the day. If the mirrors are shielded from sunlight 
between the exposures of photographs, and if the exposures 
are made as short as possible, excellent results can be 
obtained at this time, and in the late afternoon, not long be- 
fore sunset. It has been found advantageous to direct a 
strong blast of air on the surfaces of the mirrors, by means 
of electric fans, during the exposures of the photographs and 
the intervals between them. 

It must be understood that the precautions mentioned are 
necessary only when it is desired to secure the finest possible 
definition of the solar image. When such precautions are 
used, the average photographs taken during the summer in 
the early morning with the Snow telescope and a temporary 
spectroheliograph are but little inferior to the best photo- 
graphs, secured on only a few days in the year, with the 40- 
inch Yerkes telescope and the Rumford spectroheliograph. 



138 Stellar Evolution 

The best photographs taken on Mount Wilson are distinctly 
superior to the best secured in our work with the Kumford 
spectroheliograph. It must not be supposed that no work 
can be done with the Snow telescope except under the con- 
ditions stated. As a matter of fact, very fair photographs 
can be obtained with the spectroheliograph at almost any 
time during a cool day, and in the early morning and late 
afternoon hours of a hot day without wind. It is only 
necessary to arrange the daily programme of observations so 
that the spectroheliograph, which requires the finest defini- 
tion, is used during the period when the seeing is best. 
Photographic work on the spectra of sun-spots follows, and 
after this is completed the conditions are entirely satisfactory 
for various other observations, such as bolographic work on 
the absorption of the solar atmosphere, etc. Some of the 
results obtained with the Snow telescope will be illustrated 
in subsequent chapters. 

From laboratory tests, it appears that the distortion of 
mirrors in sunlight is chiefly due to actual bending of the 
glass, the front surface, expanded by the heat, becoming 
convex and the rear surface concave. Radiation from an 
electric heating coil, placed a short distance behind a mirror, 
restores its figure, but not perfectly. A much better way of 
heating the back of a mirror is by reflecting sunlight upon 
it. Perhaps the best plan, however, is merely to increase 
the thickness of tlie glass mirrors (p. 235). 



CHAPTER XVI 
SOME USES OF SPECTROHELIOGRAPH PLA.TES 

The necessity of designing the Rumford spectroheliograph 
for use as an attachment of the Yerkes telescope interfered 
somewhat with its efficiency. Under good conditions it gives 
excellent results, but the limitations of aperture, and the 
difficulty of securing perfect equality in motion of plate and 
solar image, are sometimes apparent in the photographs ob- 
tained with it. Fortunately, the case was different with the 
Snow telescope. It was possible here to adopt the most 
satisfactory form of spectroheliograph, in which the instru- 
ment is moved as a whole, while the image of the Sun and 
the photographic plate are stationary. The first spectrohelio- 
graph of this type was constructed in 1893 and employed 
in attempts to photograph the solar corona without an 
eclipse, from the summit of Mount Etna. For all instru- 
ments of moderate dimensions, motion of the spectrohelio- 
graph as a whole appears to be preferable to any mechanical 
contrivance for moving the plate and solar image in syn- 
chronism. 

A photograph of the spectroheliograph, mounted for use 
with the Snow telescope, is reproduced in Plate LXI. A better 
idea of the general design may be obtained from Plate LXII, 
which shows the spectroheliograph in our instrument shop 
before it was completed. It consists essentially of a massive 
cast-iron base, bearing four short V-rails at its four corners, on 
which the moving part of the instrument is carried by four 
steel balls. The cast-iron platform which bears the slits and 
optical parts has four inverted A-i*ails, which rest on the 
steel balls, but almost its entire weight is supported by 

139 



140 Stellar Evolution 

mercury, in three tanks formed by subdivisions in the base 
casting. Wooden floats extend from the lower surface of 
the iron platform into these tanks, reducing to a minimum 
the amount of mercury (about 560 pounds) required to up- 
hold the instrument. The motion of this platform with re- 
spect to the fixed solar image and photographic plate is pro- 
duced by either one of two screws of different pitch, driven 
by an electric motor arranged to give a perfectly uniform 
speed. 

The collimator slit, on which the solar image is formed, 
is shown on the right of Plate LXI. On account of the large 
size of the solar image, which is about 6 . 7 inches in diameter, 
the slit is 8^ inches long. After passing through the slit 
the light falls upon a large collimating lens 8 inches in diam- 
eter, which renders the rays parallel. They then meet a 
silvered glass mirror, from which they are reflected to the two 
prisms, of 63^° angle. After being dispersed by the prisms 
the rays strike the 8 -inch camera lens, which forms an image 
of the spectrum on the camera slit (shown near the center of 
Plate LXI). The optical train thus resembles that of the 
Rumford spectroheliograph, but the lenses and prisms are 
so much larger that no light is lost from the circumference 
of the solar image. 

On account of the great curvature of the spectral lines 
produced by such prisms, it would be necessary to employ a 
highly curved camera slit, in case an ordinary straight slit 
were used to admit the light from the Sun. In this event 
the resulting photograph would be greatly distorted, because 
points lying along a straight line on the Sun would appear 
along a curved line in the photographs. Thus the image, 
instead of being circular, would be shaped somewhat like 
an apple, greatly flattened on one side. By dividing the 
curvature evenly between the two slits the distortion is 
eliminated and the photograph is made circular. 



Uses of Spectroheliogkaph Plates 141 

The actual operations in making a photograph of the 
flocculi with one of the calcium lines are as follows: An 
electric arc, the carbons of which have been moistened with 
a solution of calcium chloride, is mounted in front of the 
collimator slit. The bright H and K lines are easily visible 
in the spectrum of such an arc, although the same region of 
the solar spectrum is difficult to see distinctly. By means of 
a micrometer screw, the camera slit is made to coincide with 
one of the lines. Thus the only light which can reach the 
photographic plate is that of calcium vapor. Up to this time 
the mirror of the coelostat has been shielded from the Sun 
by a canvas screen, in order to protect it from distortion. 
After the photographic plate has been placed in position in 
its support in front of the camera slit, the canvas screen is 
removed and the solar image brought to a sharp focus on the 
collimator slit, by moving the concave mirror of the Snow 
telescope. The slide of the plate-holder is then drawn and 
the electric motor started. The screw, driven by the electric 
motor, then causes the entire spectroheliograph to move at a 
slow and uniform rate, so that the collimator slit passes over 
the solar image and the camera slit moves across the photo- 
graphic plate. 

If it is desired to take a photograph with a hydrogen line, 
instead of a calcium line, the prisms and mirror are adjusted 
until the line in question falls upon the camera slit, when 
the exposure is made as before. 

In the daily programme of observations at least one photo- 
graph with the Hj line of calcium, showing the faculae and 
low level calcium vapor; one with the Hg line of calcium, 
showing the flocculi at a higher level; one with the H'y line 
of hydrogen; and one with an iron line, are made in the 
early morning and again, if circumstances permit, in the late 
afternoon (Plates LXIII-LXVII). Since the weather is 
clear day after day through the summer and autumn months 



142 Stellar Evolution 

(on 112 consecutive days in the summer of 1907), and not 
infrequently during the rainy season, the instrument thus 
yields a large number of plates, suitable for the comparative 
study of the flocculi. 

Photographs of the prominences are also made daily, when 
circumstances permit. These are used to determine the 
changes in number and total area of the prominences during 
the Sun-spot period. 

In the establishment of an observatory much remains to 
be done after successful photographs of astronomical phe- 
nomena have been obtained. Indeed, although the work of 
organization must be far advanced before photographs can 
be secured, the most important steps are still to be taken. 
For an astronomical photograph, while it may yield much 
new information from casual examination, is to be regarded 
as a document of great value, worthy of prolonged investi- 
gation. Every photograph of the Sun, for example, repre- 
sents its changing phenomena as they were at the moment 
of the exposure, under conditions which will never be exactly 
repeated. The best methods of obtaining from photographs 
all the knowledge they are capable of conveying are to be 
arrived at only after the fullest consideration of the possi- 
bilities. 

In chap, xi the most striking characteristics of the floc- 
culi have been explained and illustrated. We must now 
consider how these objects may be systematically studied, in 
such a way as to contribute to our knowledge of the solar 
constitution. The most obvious peculiarity of the flocculi, 
apart from their change in form, is their motion across the 
Sun's disk. This is due to the solar rotation, which was 
first discovered through the daily motion of sun-spots. It 
is remarkable that the spots do not move as they would if 
they were fixed to the surface of a solid sphere. Spots in 
different latitudes move with different angular velocities, and 



Uses of Spectroheliograph Plates 143 



exhibit wiiat is called the "equatorial acceleration;" i. e., 
spots near the Sun's equator complete a revolution in much 
shorter time than those in higher latitudes. At the equator 
the rotation period is about twenty-five days. At 10° north 
or south latitude the period is several hours longer, and at 
45° it is about twenty-seven and a half days. The faculae, 
according to results obtained by Stratonoff and others, follow 
the same general law. Spectroscopic observations, based on 
an application of Doppler's principle show that the motion 
is not confined to the spots and faculae, but is also shared 
by the layer of metallic vapors (the "reversing layer") which 
lies just above the photosphere, and produces the dark lines 
of the solar spectrum by absorption of the white light 
coming through it from below. It thus becomes interesting 
to inquire whether the calcium flocculi, which we suppose to 
be clouds of luminous vapor lying at an elevation of several 
thousand miles above the photosphere, show a similar law 
of rotation. 

The method employed to determine the rotation period of 
the spots is to measure their latitude and longitude, referred 
to the center of the Sun, on plates taken at intervals of one 
or more days, and in this way to ascertain the change of 
longitude of the same spot in twenty -four hours. By thus 
obtaining the velocities of spots in different latitudes the law 
of rotation can be derived. In considering the methods of 
measuring the latitude and longitude of a spot, we must 
remember that the plane of the Sun's rotation is inclined at 
an angle of about 7° with that of the Earth's orbit. The 
Earth passes through the nodes (the intersection of this 
plane with the ecliptic) about June 3 and December 5, and 
only on these dates do the spots appear to move in straight 
lines across the disk. The angle between the Sun's axis and 
the north and south line in the sky (called the "position 
angle" of the Sun's axis) varies about 53° in the course of 



144 Stellar Evolution 

the year — about 26^° each side of zero. It is thus evident 
that in determining the latitude and longitude of a spot by 
ordinary methods of measurement considerable calculation 
will be required. The process employed at Greenwich, on 
the direct photographs of the Sun obtained there, is to meas- 
ure the distance of the spot from the center, and the angle 
between the Sun's axis and the line joining the spot with 
the center of the disk. As the inclination of the Sun's axis 
is known for every day in the year, it then becomes possible 
to calculate the latitude and longitude of the spot. 

This method is very satisfactory when a comparatively 
small number of objects are to be measured on each plate, 
which is the case with sun-spots. But the flocculi are so 
numerous, and offer so many points suitable for measure- 
ment, that the calculations required for each spectrohelio- 
graph plate would be very extensive. In seeking to find 
some simple method of abridging these calculations, it 
appeared that the solar photograph might be projected 
upon the surface of a globe ruled with meridians and 
parallels 1° apart. The axis of the globe being set at the 
inclination corresponding to the date of the photograph, it 
should then be possible to read off the latitude and longitude 
directly, by estimating the position, in tenths of a degree, 
of the flocculus in question, with reference to the nearest 
meridian and parallel (Plate LXIX). As the longitude of 
the center of the Sun's disk is tabulated for each day in the 
year, no calculations would be necessary, except to add or 
subtract this longitude in the case of each of the readings. 

This method proved so satisfactory, when used at the 
Yerkes Observatory in measuring the Kenwood photographs, 
that it was afterward adopted, in perfected form, in the 
Computing Division of the Solar Observatory. The new 
globe-measuring machine, or "heliomicrometer," is illus- 
trated in Plate LXX. Two 4-inch telescopes, shown in the 



Uses of SpecteohelioCxRaph Plates 145 

upper part of the cut, are pointed toward two plane silvered 
glass mirrors thirty feet away. One of these mirrors receives 
light from the spectroheliograph plate, which is. mounted 
immediately under the right-hand telescope and illuminated 
by incandescent lamps from behind. The other receives 
light from a globe, mounted below the left-hand telescope 
and illuminated on its front surface. The images of globe 
and plate, given by the two telescopes, are brought together 
in a single eye-piece, so that the observer sees them super- 
posed. If, then, the surface of the globe is ruled with 
meridians and parallels, as in the instrument previously 
described, the positions of the flocculi can be read off by 
estimation. However, it is desired in this case to attain a 
higher degree of precision in the measurements, and to see 
small and faint flocculi to better advantage than would be 
possible if they were observed in projection against the illu- 
minated surface of the globe. Accordingly, a pair of cross- 
hairs, which can be moved over the plate in a horizontal or 
vertical direction by the observer at the eye-piece, is made 
to coincide with the object to be measured. The globe is 
then illuminated, and rotated in latitude and longitude until 
a point corresponding to the intersection of the equator and 
the central meridian falls exactly upon the cross-hairs. A 
circle, which can be read by the observer at the eye-piece, 
then shows the angle through which the globe has been 
turned in latitude. A second circle gives the distance in 
longitude from the center of the Sun. It is, of course, to be 
understood that the axis about which the globe is turned in 
measuring longitudes is set at the proper inclination for the 
date of the photograph. For less precise measurements, the 
position of the cross-hairs may be estimated with reference 
to the rulings on the (fixed) globe. 

This instrument, which was constructed in the shop of 
the Solar Observatory, has proved very satisfactory in prac- 



146 Stellae Evolution 

tlce. It has been found that the latitudes and longitudes, 
thus read off directly, are as accurate as when determined 
by measuring the plate in an ordinary measuring-machine 
and performing the necessary calculations. Since the meas- 
urements can be be made quite as rapidly on the heliomi- 
crometer as on the other machine, all the time required to 
make the calculations is saved. Thus one observer can 
measure a great number of flocculi, and the services of sev- 
eral computers are rendered unnecessary. 

A discussion of the measurements made in this way shows 
that the flocculi follow a law of rotation similar to that which 
governs the spots and faculae. It will require some time to 
learn whether the velocities of the flocculi differ appreciably 
from those of the spots. It appears probable, however, from 
results thus far obtained, that the flocculi move with about 
the same velocity as the faculae. This would be a natural 
result, since, as already explained, the vapors of the flocculi 
probably rise from the faculae, and lie immediately above 
them. 

The importance of providing for the closest possible cor- 
relation between all of the investigations of the Solar Obser- 
vatory has already been mentioned. For this reason studies 
of the solar rotation should be made with reference to other 
solar work. The motions of individual flocculi frequently 
differ considerably from the average motions of the flocculi in 
the same latitude. Such differences, in many instances, are 
doubtless similar to those observed in the case of sun-spots, 
where they are related to the spot's activity, which varies 
greatly during the course of its development. However, the 
daily motion of a flocculus may also depend upon its height 
above the photosphere, and this may vary from day to day. 
It thus becomes desirable to learn whether differences in the 
height of flocculi can be detected and actually measured. 
For example, do the hydrogen flocculi lie at an average level 



Uses of Spectroheliograph Plates 147 

in the solar atmosphere above or below that of the calcium 
flocculi, and, if so, do they show differences of rotational 
velocity that may depend upon this fact? 

It has already been explained, in chap, xi, that the cal- 
cium flocculi photographed when the bright H„ or Kg line 
is employed probably lie above the bright objects of similar 
form, but somewhat smaller area, which are photographed 
when the slit is set on the broad H, or K^ band. It is not so 
obvious, however, that the average level of the hydrogen floc- 
culi is above that of the Hg and K^ calcium flocculi, but this can 
be determined by accurate measurements. The forms of the 
dark hydrogen flocculi, as already remarked, closely resemble 
those of the bright calcium flocculi, though in many cases 
there are important differences (Plates LXXI and LXXII).^ 
With the aid of the stereocomparator, an instrument manu- 
factured by the Zeiss Optical Company for the purpose of 
making accurate comparisons of photographs, it is possible 
to observe a hydrogen photograph in superposition upon a 
calcium photograph, taken within so short an interval of time 
that no appreciable change occurred on the Sun between the 
exposures. With the monocular eye-piece of the instrument 
the two photographs, in precise superposition, are observed in 
quick succession. For this purpose a device is used which 
permits the eye to see one of the plates, and, immediately 
afterward, the other. If a micrometer wire is set on a cal- 
cium flocculus lying near the edge of the Sun, and the image 
of the corresponding hydrogen flocculus is then brought into 
view, it is found to be displaced slightly away from the center 
of the disk. This is not true of all the hydrogen flocculi. 
On the average, however, these dark hydrogen clouds seem to 

1 These photographs were separated by an interval of 2ii 26m, during which time 
the changes in the forms of the flocculi would not ordinarily be sufficiently marked 
to interfere with the general comparison of the more conspicuous features. In this 
case, however, the changes may have been rapid, since the numerous bright flocculi 
near the spot indicate great eruptive activity. For the accurate comparison of 
details, the photographs must be taken simultaneously. 



148 Stellak Evolution 

be displaced in this way, by an amount representing a height 
of some 1,500 miles above the corresponding calcium clouds/ 
It will therefore be interesting to determine at some future 
time whether the rotational velocity of the hydrogen flocculi 
differs appreciably from that of the calcium flocculi. 

An important step in the interpretation of spectrohelio- 
graph plates will be made when it can be ascertained whether 
anomalous dispersion plays any part in producing the phe- 
nomena recorded by them. Our present views as to the 
nature of Sun-spots, prominences, and other solar phenomena 
are based on the assumption that their light reaches us along 
nearly straight lines. If the pressure in the region through 
which the rays pass is low, this may be essentially true for 
white light. But we know that light of about the same 
wave-length as that of an absorption line in the spectrum, is 
bent far out of a straight path when it passes through the 
vapor to whose absorption the line is due. The conse- 
quences of this fact have led Julius to develop a new solar 
theory, based on the supposition that all metallic vapors at 
any given distance from the Sun's center are completely 
mixed, but not of uniform density throughout. Under these 
circumstances the chromosphere, prominences, and flocculi 
would not exist as we see them, but such appearances might 
be caused by anomalous dispersion of light passing out 
through the vapors from the interior of the Sun. A series 
of investigations, involving solar, stellar, and laboratory work, 
is being carried out on Mount Wilson for the purpose of 
testing this theory. 

The rotation periods of sun-spots may depend upon their 
level, and this raises the old question as to the position of 
these objects with respect to the photosphere. According 
to the common view sun-spots are saucer-shaped cavities in 
the photosphere. This idea is based upon the observations 

1 This result must be checked on photographs taken simultaneously. 



Uses of Spectroheliograph Plates 149 

of Wilson, who found that when a spot is carried toward the 
limb by the solar rotation, the penumbra, on the side toward 
the center of the disk, is reduced in apparent width, as it 
would be, on account of its inclination to the line of vision, if 
it sloped downward toward the umbra. The best modern 
results do not offer any certain confirmation of this view, and 
thus render necessary an appeal to some independent test of 
the question. Ten years ago it was pointed out by Frost 
that the heat radiation of a spot, as compared with that of the 
neighboring photosphere, increases as the spot approaches 
the limb. From this it was naturally concluded that the spot 
must lie above the photosphere, at such a level as to escape 
the influence of the low-lying absorbing veil, which so greatly 
reduces the intensity of the photospheric light at the solar 
circumference. It has recently been found, however, that 
sun-spots radiate a much smaller proportion of violet light 
than the photosphere. As violet light is always reduced by 
an absorbing atmosphere in much larger proportion than 
light of longer wave-length, it follows that the observed 
effect would be seen in the case of sun-spots, even if they 
were at the same level as the photosphere. To remove 
the difficulty it is only necessary to confine the comparative 
measures to a single color, rather than to use the total radia- 
tion, comprising light of all wave-lengths. 

The spectroheliograph affords a simple means of accom- 
plishing this. It is employed to make photographs of a sun- 
spot and the surrounding photosphere on various dates, corre- 
sponding to the changing position of the spot on the solar 
disk. In making these photographs the camera slit is set, not 
on any of the spectral lines, but on a space between the lines, 
preferably in the yellow or red, since the influence of extra- 
neous light will be least marked in this region. On account 
of the darkness of the spot, which would require an exposure 
about six times as long as that for the photosphere to give a 



150 Stellar Evolution 

photograph of equal intensity, it is desirable to decrease the 
intensity of the photospheric light by a dark glass, placed over 
the slit, but so arranged as not to reduce the light from the 
sun-spot. In this way the spot and photospheric light can be 
compared from day to day, by means of photometric meas- 
urements. The same method can be employed to measure 
the level of the flocculi. Such work is now in progress at the 
Solar Observatory, in conjunction with the other investiga- 
tions already mentioned. Since the level of a spot may 
affect its temperature, and therefore its spectrum, an attempt 
will be made to correlate this work, not only with determina- 
tions of the spot's motion, but also with the spectroscopic 
observations described in chap, xvii. 

These few examples may suffice to give an idea of the 
character of the work done with the spectroheliograph of the 
Snow telescope. The vertical motion of the calcium vapor 
in the flocculi; the manner in which it flows horizontally 
over sun-spots ; the relationship, in point of development, of 
flocculi to spots ; and other similar matters, are also studied 
systematically. It may also be added that the area of the 
flocculi is measured on each day's plates, since it serves as an 
index to the Sun's activity, which may prove important when 
considered in its bearing on possible variations of the solar 
radiation and their effect on terrestrial phenomena. 



CHAPTER XVII 

A STUDY OF SUN-SPOTS 

It has already been remarked (p. 69) that sun-spots, 
though apparently much darker than the photosphere, are, in 
reality, brilliantly luminous objects. Though they thus ap- 
pear dark merely by contrast, the cause of their reduced 
brilliancy has given rise to much discussion. Some of the 
most recent theories have maintained that sun-spots are so 
much hotter than other parts of the solar surface that the 
photospheric clouds, due to condensation of the vapors rising 
from the Sun's interior, cannot form at these points. One of 
Lockyer's arguments in support of his hypothesis that the 
terrestrial elements are dissociated at the high temperature 
of the stars is based upon the view that at times of sun-spot 
maxima the spots are too hot to permit certain of the terres- 
trial elements to exist in them. This conclusion was founded 
upon a long series of observations of certain lines in the 
spectra of sun-spots. The spot spectrum differs from the 
solar spectrum in the fact that some of the solar lines are 
strengthened or widened, some are weakened, and many are 
unchanged (Plate LXXIV). The number of lines whose 
intensities are thus altered amounts to many hundreds; 
indeed, if the fainter lines are taken into account, to several 
thousands. Lockyer's observations consist in recording, on 
every clear day, the "twelve most widened lines" in the 
spectra of spots then visible on the Sun. His results 
seemed to indicate that at sun-spot minima the most widened 
lines represent known substances; while at sun-spot maxima 
many of these give place to unknown lines, which he attrib- 
uted to unknown substances produced by dissociation of the 

151 



152 Stellae Evolution 

elements at the high temperature assumed to be character- 
istic of periods of greatest solar activity. Some of his later 
papers favor the view that sun-spots^ possess a lower tem- 
perature than would thus be indicated, and he may therefore 
have decided to abandon the conclusions based on his earlier 
spectroscopic observations. 

In spite of these results, and of all the theories which 
attribute high temperature to sun-spots, the more common 
opinion has been that they are regions of reduced tempera- 
ture. This view has been based partly upon their decreased 
brightness, as compared with the photosphere, and partly 
upon the presence in their spectra of certain bands which, 
though unidentified, were supposed to represent molecules 
that cannot exist at the high temperature of the Sun. Accu- 
rate knowledge of these bands, however, was almost entirely 
lacking, on account of their faintness and the extreme diffi- 
culty of observing them visually. 

It seemed probable that progress in this department of 
solar research might be expected to result from the success- 
ful application of photography to the study of spot spectra. 
Experiments made with this object in view at the Kenwood 
Observatory showed some of the principal widened lines, 
but failed to give the details needed for satisfactory work. 
These results were surpassed by photographs made by Young ■ 
with the 23-inch Princeton refractor, but here also the need 
of more powerful instrum^ental means seemed to be apparent. 
The Kenwood experiments were continued with the 40-inch 
Yerkes telescope, and some of the "band lines," first observed 
visually by Maunder, were photographed, in addition to many 
of the widened lines. However, there was reason to believe 
that much better results could be obtained with the aid of a 
long-focus grating spectrograph, capable of photographing 

1 Because of the great strength of the titanium lines in Arcturus. 



A Study of Sun-Spots 153 

the spectrum on a large scale. Further work was therefore 
deferred until it could be taken up with the Snow telescope 
and a powerful Littrow or auto-collimating spectrograph. 

This spectrograph is of a very simple type. The image of 
the Sun is formed on a slit, s, through which the light passes to 
a 6-inch collimating objective, o, of 18 feet focal length, which 
renders the rays parallel (Fig. 6). The rays then fall upon 
a plane grating, r/, which diffracts them into a series of 
spectra. Light from a portion of one of these spectra returns 
to the objective, o, which forms an image of the spectrum on 

M . -f' 

\ ° 

FIG. 6 
Path of Rays in Littrow Spectrograph 

a photographic plate, p, standing just above the slit. In 
order to form the image at this point the grating must be 
slightly inclined backward, so as to send the beam upward. 

This instrument, as mounted for use with the Snow tele- 
scope, is shown in Plate LXXIII. As the tube of the spec- 
trograph stands immediately above the spectroheliograph, a 
section of it can be rotated out of the way, to permit access 
to the prism-train of the latter instrument. When the 
spectrograph is to be used instead of the spectroheliograph, 
the concave telescope mirror is moved north through a suffi- 
cient distance to transfer the focal plane from the spectro- 
heliograph slit to the spectrograph slit. Then, by inclining 
the mirror backward through a small angle, the solar image 
is raised to the proper height. After final focusing, a sun- 
spot is brought exactly upon the slit with the aid of slow- 
motion electric motors, connected with the concave mirror 
and controlled from a point near the focal plane. 

In photographing the spectrum of a sun-spot, all light is 



154 Stellar Evolution 

excluded from the spectrograph except that which comes from 
the umbra. This is done by covering all of the slit except 
a small portion at the center. The dispersion of the second 
or third order of the grating is usually employed. After this 
exposure has been completed, the center of the slit is covered 
and light from the photosphere admitted on each side. This 
gives a narrow photograph of the spot spectrum between 
two strips of solar spectra (Fig. 1, Plate LXXIV). 

Casual examination of the spot spectra thus recorded is 
sufficient to show that the problem of interpreting them is 
not a simple one. If we consider, for example, the lines of 
some single element represented in the spot, we find that they 
are not all affected alike. Some are greatly strengthened, 
or perhaps attended by broad, faint wings. The former effect 
is so very pronounced, in certain cases, that lines wholly 
invisible in the solar spectrum are among the most conspi- 
cuous of the spot lines. Some of the solar lines, on the con- 
trary, are greatly weakened, or entirely absent in the spot 
spectrum. Finally, there are many spot lines of unchanged 
intensity. Examples of most of these phenomena are illus- 
trated in Plates LXXIV and LXXVI. 

In order to interpret such results, it is necessary to have 
recourse to laboratory experiments. It might be supposed 
that the required knowledge of terrestrial spectra would be 
available in the literature of spectroscopy. This, however, 
is not the case. It is true that the lines in the spectra of 
most of the elements have been measured, and many experi- 
ments have been made on the changes in spectra produced 
by varying the conditions under which the vapors emit their 
radiations. It usually happens, however, when one attempts 
to apply published results to the interpretation of solar 
phenomena, that the data required for the solution of the 
particular problem in hand are lacking. Pressure, for 
example, is known to displace spectral lines toward the red, 



A Study of Sun-Spots 155 

and the actual shifts of certain lines of several different ele- 
ments have been measured. But these form a very small per- 
centage of the total number of lines in the spectra of these 
substances, and the shifts of any lines that happen to be 
under investigation are rarely found in the published tables. 
The same may be said of the effect of temperature on spectra. 
It has long been known that a reduction of temperature 
increases the relative brightness of certain lines, decreases 
that of others, and is without effect on the rest of the spec- 
trum. Indeed, it was even known that some of the iron lines 
which are prominent at low temperatures are among the more 
conspicuous lines of spot spectra. But these instances were 
so few and scattered that no safe inferences could be based 
upon them. Moreover, it had not been definitely proved that 
these changes of relative intensity could actually be produced 
by temperature alone. Most of the experiments showing 
variations of spectra have involved the use of electric dis- 
charges, where causes are at work which might have a far 
greater influence than temperature change on the character 
of the spectra. Examples might easily be multiplied to show 
that the study of solar and stellar physics cannot be carried 
on effectively without a constant appeal to laboratory experi- 
ments, planned with special reference to the needs of the 
particular problem under investigation. 

For this reason much stress has been laid in the equip- 
ment of the Solar Observatory upon the provision of suitable 
laboratory facilities. It seemed essential, in designing the 
spectroscopic laboratory on Mount Wilson, not only to in- 
clude a considerable number of light-sources, which could 
be examined under various conditions of temperature, pres- 
sure, etc., but also to arrange them in such a way that the 
appeal to one or the other condition could be made without 
the delays ordinarily experienced when apparatus must be 
specially set up for a certain investigation. In the desired 



156 Stellar Evolution 

plan the apparatus must be always ready, needing only the 
turning-on of an electric current, or the adjustment of a 
mirror, to bring it into action. It is not so much a question 
of the saving of time, which the provision of these means 
undoubtedly offers, as it is of the greatly increased efficiency 
of the working programme thus rendered possible. The 
immediate imitation in the laboratory, under experimental 
conditions subject to easy trial, of solar and stellar phe- 
nomena, not only tends to clear up obscure points, but pre- 
pares the way for the development along logical lines of 
the train of reasoning started by the astronomical work. 
Questions are constantly arising which, if partially or wholly 
answered by suitable laboratory experiments, may modify 
in an important way the daily programme of astronomical 
observations. 

The arrangement of the apparatus in the spectroscopic 
laboratory of the Yerkes Observatory has already been 
described (p. 107). At the Solar Observatory an improved 
plan has been adopted. Instead of a circular wooden table, 
an annular concrete pier is employed, giving space on the 
inner wall for the various switches used to control the cur- 
rent supplied to the different sources, and also permitting 
the observer to inspect any light-source from the direction 
of the plane mirror at the center of the pier. Instead of a 
single plane mirror, two are provided, capable of rotating 
independently of one another about the same vertical axis. 
When the Littrow spectrograph is used to photograph the 
spectrum of any of the light-sources, only the lower plane 
mirror is in action. By setting this at the proper angle, 
light from any source on the annular pier can be sent to 
a concave mirror (seen near the middle of Plate LXXV), 
which forms an image on the slit of the Littrow spectro- 
graph. If low dispersion, rather than high dispersion, is 
required, a one-prism quartz spectrograph is used. Again, 



A Study of Sun-Spots 157 

for the special study of certain lines under the highest resolv- 
ing power, particularly in investigations of the Zeeman effect, 
an echelon spectroscope is used. In either case the concave 
mirror is tipped back at a small angle, so as to return the 
light to the upper plane mirror, from which it is reflected to 
the slit of one of these instruments. In Plate LXXV the 
quartz spectrograph may be seen just above the concave 
mirror, while the echelon spectroscope stands on the extreme 
right, near the end of the room. The Littrow spectrograph, 
which is ordinarily employed, is similar in type to the spec- 
trograph used with the Snow telescope. The rectangular box 
which carries the slit and plate-holder of this instrument is 
show'U on the pier in the lower left corner of Plate LXXV. 
The following apparatus stands on the annular pier: the 
first instrument on the right is a powerful electro-magnet, 
used for the study of the Zeeman effect — i. e., the influence 
of a magnetic field in separating spectral lines into several 
components. For example, in the spectrum of a spark passing 
between iron terminals most of the lines appear single, even 
when observed with the great resolving power of an echelon 
spectroscope. If, however, the spark is placed between the 
poles of a powerful magnet, the effect of the magnetic field 
is to break each line up into several components. It would 
take us too far away from our immediate subject to discuss 
the theoretical questions which underlie these phenomena. 
It may be said, however, that by observing whether certain 
lines behave similarly under the influence of a magnetic 
field, we can tell whether they would be expected to act 
together in the Sun. It is not a question here of detecting 
magnetic phenomena in the Sun, since most careful study 
has not revealed any evidence of solar magnetic fields capable 
of affecting the appearance of the spectral lines. Neverthe- 
less, the method provides an arbitrary means of picking out 
certain groups of lines, which may be so intimately related 



158 Stellar Evolution 

to one another that we should expect them always to behave 
alike when observed in the Sun or stars. 

In the illustration a mercury tube is suspended between 
the poles of the magnet and connected by heavy pressure 
tubing with a duplex vacuum pump, by which the pressure 
of the mercury vapor, illuminated by the discharge of an 
induction coil, can be reduced as desired. The current 
required for the magnet is supplied from a large storage 
battery in an adjoining building. This battery is the prin- 
cipal source of current for most of the apparatus on the 
annular pier; an alternating current, required for certain 
experiments, is obtained from a generator in the power-house. 

It would be tedious to describe in detail all of the appa- 
ratus. It includes arrangements for studying the spark 
spectra of metals in air and in liquids; arc spectra in gases 
at high or low pressure; flame spectra, for which a Bunsen 
burner and an oxyhydrogen blow-pipe are required; vacuum 
tube spectra; etc. A small electric furnace permits the phe- 
nomena of anomalous dispersion to be observed in the vapors 
of sodium and other metals which melt at low temperatures. 
The auxiliary apparatus includes a special pump capable of 
compressing gases up to pressures of three thousand pounds 
to the square inch; an induction coil, giving a 16-inch 
spark; X-ray apparatus for the study of the effect of X-rays 
on the radiation of gases and vapors; a small heliostat, to 
supply sunlight ; etc. All of the work on the solar image 
is done in the Snow telescope house, but sunlight is fre- 
quently required in the laboratory, to give a solar spectrum 
for comparison with the laboratory spectra. 

Let us now return to the problem of explaining the 
strengthening and weakening of the solar lines in sun-spot 
spectra. As already remarked, there was reason to suspect 
that reduced temperature might be the effective cause of 
these changes. Accordingly, the spectrum of iron was 



A Study of Sun-Spots 159 

pliotographed by Gale and Adams in the electric arc, first 
with a large current (15 amperes), and then with a small 
current (2 amperes). It was found that most of the lines 
that are strengthened in spots are relatively strengthened in 
the 2-ampere arc, while most of the lines that are weakened 
in spots are also w^eakened in this arc. Furthermore, the 
majority of the lines showed no change of intensity, which 
is also the case with most of the iron lines in sun-spots. 
Similar results were obtained with titanium, vanadium, chro- 
mium, manganese, and other metals represented in spots. 

The next question was to determine whether the metallic 
vapors in the 2-ampere arc are certainly cooler than in the 
30-ampere arc. This is by no means an easy thing to decide, 
on account of various complicating elements that may not 
appear at first sight. However, it was a simple matter to 
compare the spectrum of the long flame which extends out 
from the arc with that of the core of the arc between the 
carbon poles. As the outer part of the flame is undoubtedly 
much cooler than the core of the arc, the effect of decreased 
temperature should be apparent here. The results confirmed, 
in the most complete manner, those obtained by reducing 
the current. In other words, in passing from the hot core 
of the arc to the cooler flame, changes in the relative inten- 
sities of the lines of the various metals, similar to those 
observed in comparing the solar spectrum with the sun-spot 
spectrum, were found. It thus seemed probable that the 
modified relative intensities of the lines in spots might be the 
result of a local reduction in temperature of the solar vapors. 

However, it is not known precisely what part electrical 
phenomena in the arc may play in producing the character- 
istic radiations of the vapors. Indeed, opinions have differed 
so much on this subject that some of the ablest physicists 
ascribe the observed line intensities entirely to the electrical 
conditions of the arc, and do not admit that temperature 



160 Stellar Evolution 

changes can have any influence upon them. Thus the 
results so far obtained would not be accepted as proof that 
the spot vapors are at a lower temperature than the corre- 
sponding vapors in the Sun's reversing layer. It remained 
to be seen whether simple reduction of temperature, under 
conditions which excluded any possible influence of electrical 
effects, would be competent to change the relative intensities 
of the lines in the same way as passage from the core to the 
flame of the arc had done. 

The simplest way of testing this was to inclose the metal 
in question within a carbon or graphite tube (chosen because 
of its power to withstand very high temperatures), and to 
heat this tube by a powerful electric arc playing on its outer 
walls. Under these conditions, since the vapors are not 
observed within the electric arc, but are separated from the 
flame of the arc by the walls of the carbon tube, it should 
be possible to determine the effect of change of temperature 
on the relative intensities of the lines. 

As the dynamo on Mount Wilson was not adequate to 
supply the electric power (50 kilowatts) desired for this 
work, the furnace was erected in the Pasadena laboratory 
of the Solar Observatory. As in an electric furnace used 
by Moissan, the arc was produced between two large carbon 
poles, in a box with carbon walls, surrounded by a large mass 
of magnesite, inclosed in a sheet-iron case. Running longi- 
tudinally through the carbon box, and between the poles of 
the arc, a carbon tube containing the metal was placed. This 
tube extended out through the walls of the furnace, so that 
light from the hot vapors seen through its open end could 
be focused on the slit of a Littrow spectrograph of 18 feet 
focal length (similar to the one used with the Snow telescope 
in photographing spot spectra). 

With this furnace it did not prove to be possible to 
vaporize titanium and vanadium, but the test was made for 



A Study of !Sun-!Spots 161 

chromium and iron. The relative intensities of the lines of 
these metals were found to be very nearly the same as in 
the flame of the arc. In other words, the lines which are 
strengthened in passing from the core of the arc to the flame 
are also strengthened in passing from the core of the arc to the 
electric furnace. Moreover, even after the arc which heated 
the carbon tube in the furnace had been extinguished, the 
still glowing vapors continued to give a spectrum in which 
the lines strengthened in sun-spots were relatively strong. 

But the proof is not yet complete. For, with the facili- 
ties available, it was not possible to vary the temperature in 
the furnace through a sufficient range to produce undoubted 
changes in the relative intensities of the lines. Therefore it 
might be argued that the increased intensity in the core of 
the arc of some lines, and the decreased intensity of others, 
are due to electrical phenomena, and not to increased tem- 
perature. The inference was strong that reduced tempera- 
ture was the deciding factor in determining the relative 
intensities of the lines, since it is common to the flame of 
the arc and to the furnace, and since electrical effects were 
excluded in the latter. But the laboratory work cannot 
furnish an absolute proofs unless it should become possible, 
through increase in the temperature of the furnace, to pro- 
duce spectra in which the relative intensities of the lines are 
the same as in the case of sun-spots. Experiments are now 
in progress with this end in view. 

Fortunately, how^ever, there are other sources of infor- 
mation to which we may appeal. In the reversing layer, 
oxygen exists in the presence of such substances as iron and 
titanium. Now, it is well known that this can be true only 
under conditions of very high temperature. Hence, if the 
metallic vapors in sun-spots are actually cooler than the 
vapors outside of spots, the reduction in temperature may 
be sufficient to permit the oxygen to enter into combination 



162 Stellar Evolution 

with some of the metals present. Titanium, oxide, in partic- 
ular, is capable of resisting a very high temperature, which 
would immediately dissociate an oxide of iron. Is there any 
evidence, then, that titanium oxide exists in sun-spots? 

Thanks to the excellent photographs of spot spectra ob- 
tained with the aid of the Snow telescope, this question is 
easily answered. Titanium oxide gives a very characteristic 
fluted spectrum, consisting of bands in which the numerous 
lines lie closer and closer together until they terminate in 
definite "heads." Fig. 2, Plate LXXIV, shows some of 
these titanium oxide flutings in the extreme red end of the 
spectrum, as photographed (on specially sensitized plates) 
in the outer flame of the electric arc. The photograph is 
a negative; i. e., the lines which are bright in the arc are 
shown dark, to facilitate comparison with the dark lines 
in the photograph of the spot spectrum, shown just above 
the titanium oxide spectrum. It will be seen at a glance 
that each of the heads of the fluting is represented in 
the spot, and that a great number of the fine lines which 
make up the fluting also agree in position with correspond- 
ing spot lines. The spot spectrum contains many lines 
not represented in the arc, which are due to substances 
other than titanium oxide. The arc spectrum also contains 
a few lines due to impurities, which are not present in the 
spot. Nevertheless, the general agreement is so perfect that 
the presence of the titanium oxide bands in spot spectra 
cannot be doubted. Several other bands belonging to the 
same substance are also represented in our photographs of 
spot spectra. 

The identification of these bands by Adams would seem 
to leave no doubt as to the reduced temperature of the spot 
vapors. The objection might be made, it is true, that some 
question exists as to whether these bands are actually due to 
the oxide, since there is some reason to suppose that titanium 



A Study of Sun-Spots 163 

itself is capable of producing them. However, the molecule 
which gives them rise undoubtedly differs from the atom 
which produces the line spectrum of titanium. In laboratory 
experiments the flutings become more and more conspicuous 
as the temperature is reduced, suggesting that the molecule 
is broken up at high temperatures. The absence of the flut- 
ings from the spectrum of the Sun sustains this inference. 
Moreover, Fowler, in London, has since found some of the 
green flutings in the Mount Wilson photographic map of 
the spot spectrum to be due to magnesium hydride, and 
Olmsted, on Mount Wilson, has identified some of the red 
flutings with those of calcium hydride. 

It therefore appears to be true that the vapors which con- 
stitute the umbra of a sun-spot are cooler than the corre- 
sponding vapors in other parts of the Sun. This would 
readily account for the relative intensities of the spectral 
lines and for the comparative darkness of sun-spots. But the 
cause of such a reduction of temperature is yet to be deter- 
mined. Knowledge of the comparatively low temperature of 
spot vapors at once permits us, however, to discard various 
spot theories which postulate very high temperatures, and to 
attack the question of the true meaning of sun-spots in an 
intelligent manner. 

In order to facilitate the spectroscopic study of sun- 
spots, a preliminary photographic map of the spot spectrum 
has been issued by the Solar Observatory. This consists of 
twenty-six sections, each covering one hundred Angstrom 
units of the spectrum, the whole map extending from wave- 
length 4600 to wave-length 7200. In enlarging the original 
negatives, Ellerman photographed each section on a sensitive 
plate, moved up and down (in the direction of the spectral 
lines) during the exposure. This process widened the narrow 
spot spectrum, and rendered visible many slight changes in 
the relative intensities of lines which would otherwise escape 



164 Stellae Evolution 

notice. Beside each strip of the spot spectrum the normal 
solar spectrum is given for comparison (Plate LXXVI). 

The information derived, as explained above, from solar 
and laboratory investigations applies not only to the Sun. 
If, by cooling in some degree the vapors lying within a 
limited area on the solar surface, the spectrum is changed 
in the manner illustrated in sun-spots, it should follow that 
if the entire Sun, or a star like the Sun, were cooled in the 
same degree, its spectrum would resemble that of a sun-spot. 
Our ideas of stellar evolution are based on the belief that 
stars exist in all stages of development and differing greatly 
in temperature. If our inference be correct, we should find, 
among the stars which have passed by continued cooling 
beyond the solar stage, some in whose spectra spot lines 
appear. The next chapter explains how this test has been 
applied. 



CHAPTER XVIII 
STELLAR TEMPERATURES 

The advantages of great resolving power in spectroscopic 
work have been mentioned in previous chapters. In the case 
of the Sun the amount of light at our disposal is so abundant 
that grating spectroscopes of very high dispersion can be 
used without difficulty. The degree in which the light is 
weakened by dispersion will be appreciated when it is remem- 
bered that the light entering the spectroscope through a slit 
one-thousandth of an inch in width is spread out into a spec- 
trum many feet in length. In the case of the stars, however, 
only a small amount of light is at our disposal, and for this 
reason the spectroscopes employed have always been much 
inferior in dispersion to those used for solar research. The 
interpretation of stellar spectra is thus rendered difficult, 
since several closely adjacent lines may be compressed into 
one. If, then, we are to learn the true relative intensities of 
stellar lines, in order, for example, to make certain of any 
apparent analogy with sun-spots, we must find means of 
studying stellar spectra with a dispersion as great as that 
used for solar observations. 

A difficulty which does not exist in visual observations 
has an important bearing on the nature of the spectroscopes 
required for such work. If it were possible to see the spec- 
trum of a star to good advantage, a high resolving power 
could be obtained with a spectroscope of moderate dimen- 
sions, supplied with a powerful grating. But, for two principal 
reasons, photographic methods are almost exclusively used 
in stellar spectroscopy. In the first place, except in the case 
of a few of the brightest stars, the smaller details of stellar 

165 



166 Stellae Evolution 

spectra cannot be seen, on account of tlie faintness of the 
light. In the second place, the unsteadiness of the image, 
due to atmospheric disturbances, causes the extremely narrow 
spectrum to flicker so seriously as to prevent any refined 
work. This flickering, however, has no effect upon the photo- 
graphic plate, which merely sums up all of the light it 
receives during the exposure. Moreover, by prolonging the 
exposure, a spectrum too faint to be seen can be recorded 
photographically. In all modern work of precision, there- 
fore, photographs of stellar spectra are substituted for visual 
observations. 

But the photographic plate has a granular structure, due 
to the fact that it is made up of silver grains, which can be 
separately distinguished with a microscope. On account of 
this granular structure of the plate the details of the image 
are imperfectly recorded, so that no advantage results from 
the use of high powers when examining the plate. If the 
visual image could be well seen at the spectroscope, an 
increase of magnification (attained by the use of a suitable 
eye-piece) would separate all lines within the resolving power 
of the prisms or grating. On the photographic plate, how- 
ever, the images of these lines may lie so close together that 
they appear as one, and cannot be separated by magnification. 
What is needed, in order to realize photographically the full 
resolving power of the prisms or grating employed, is a spec- 
troscope of such length that the closest lines that could be 
distinguished visually are so far separated as to be independ- 
ently recorded, in spite of the effect of the silver grains. 

The powerful grating spectrograph used by Rowland in 
his study of the solar spectrum has a focal length of 21 feet. 
Photographs made with a spectrograph of this size show 
nearly all the lines that can be separately distinguished in 
visual observations with the same instrument. Obviously it 
would be out of the question to attach such a spectrograph, 



Stellak Tempekatures 167 

or even an equally powerful one of the more compact Littrow 
type, to the end of a movable telescope tube. Moreover, 
the very high dispersion would demand, in the case of 
stars, exposures prolonged for many nights. Temperature 
changes, or the slightest flexure of the apparatus during the 
exposure, would shift the position of the lines on the plate 
and thus destroy, by producing a blurred image, all the 
advantages afforded by large spectrographs. 

Such instruments as the three-prism spectrograph of the 
Potsdam Astrophysical Observatory, the Mills spectrograph 
of the Lick Observatory, and the Bruce spectrograph of the 
Yerkes Observatory, give beautifully defined photographs of 
stellar spectra, from the measurement of which the motions 
of stars in the line of sight are determined with great pre- 
cision. For most classes of work such spectrographs could 
hardly be surpassed. Nevertheless, the necessary limitations 
of resolving power and focal length in these instruments 
prevents them from separating many of the lines resolved by 
Rowland in his studies of the solar spectrum. It is evidently 
to be greatly desired that the spectra of a few of the brightest 
stars, at least, be photographed with spectrographs as power- 
ful as Rowland's. In order to accomplish this the spectro- 
graph must be fixed in position on a massive pier, and main- 
tained at a constant temperature throughout the exposure. 

To test the feasibility of this, and to decide whether a 
spectrograph of high dispersion could advantageously be 
used with a 60-inch refiecting telescope (p. 228), a grating 
spectrograph of 13 feet focal length has been tried with 
the Snow telescope. This instrument was mounted on the 
triangular stone pier (p. 133, Fig. 5) in the spectroscope 
house of the Snow telescope. The pier is inclosed in a room 
so constructed that the fluctuations of temperature within 
it are very slight. The 6-inch Rowland plane grating was 
mounted so as to form the front wall of a cubical metallic 



168 Stellar Evolution 

box containing water. An extremely delicate thermostat, con- 
sisting of a bulb containing saturated ether vapor immersed 
in the water, caused a column of mecury to make or break 
an electric circuit if the temperature of the water varied as 
much as a hundredth of a degree. When the temperature fell 
by this amount, a relay turned on the current of two incan- 
descent lamps immersed in the liquid. The heating produced 
by the lamps raised the temperature, and the current was 
then automatically cut off. The water was constantly stirred 
by small propellers driven by an electric motor. In this way 
the grating, which is, of course, the most sensitive part of the 
apparatus, was kept at an almost perfectly constant tempera- 
ture throughout the exposure. 

Arcturus, on account of its yellowish color and the charac- 
ter of its spectrum, has long been considered to represent a 
stage of stellar development somewhat advanced beyond that 
of the Sun. As its spectral lines show its chemical com- 
position to be practically the same as that of the Sun, a 
reduction in temperature, due to cooling continued beyond 
the solar stage, should, on the hypothesis developed in the 
last chapter, cause its spectrum to resemble that of a sun- 
spot. Accordingly, the spectrum of Arcturus was photo- 
graphed with the Snow telescope and the grating spectro- 
graph. 

Because of the great dispersion, an exposure of five hours, 
which was all that could be given on a single night, was 
entirely insufficient. In fact, an exposure continued for five 
nights in succession, and aggregating twenty-three hours, 
was required. During all this time it was essential that the 
temperature of the grating remain practically constant, and 
that none of the parts of the spectrograph be displaced by 
any cause. For this reason the observer did not enter the 
constant-temperature room after the exposure was started, 
but merely brought the star to the slit of the spectrograph 



Stellak Tempekatures 169 

each night, and maintained it there, by watching the star 
image reflected from the slit jaws, and correcting any slight 
deviations in its position. The same process was repeated 
from night to night, until the exposure was completed. 

In these first experiments the possibilities of the method 
were not fairly tested, on account of some imperfections in 
the apparatus. The Snow telescope was designed for solar 
work, and is not well adapted for stellar observations. More- 
over, work in progress on the telescope house caused some 
vibration of the piers, which doubtless affected the definition. 
Nevertheless, the resulting photographs are sufiiciently good 
to show that this method, when properly carried out with 
the 60-inch reflector, should give a few stellar spectra not 
essentially inferior to the best obtained in solar work. The 
60-inch reflector will collect about six times as much light 
as the Snow telescope, and the exposure time, for the same 
dispersion, will be decreased in about this ratio. Thus the 
spectrum of Archirus should be photographed with the 
grating used for the present work in about four hours. As 
subsequent experiments with the Snow telescope showed that 
large prisms can be used to much better advantage than the 
grating for stellar spectra, this exposure time, for the same 
dispersion, will be still further reduced. In the case of the 
60-inch reflector, the dispersion will be increased sufficiently 
to make the scale of the spectrum about the same as that of 
Rowland's solar spectrum photographs. 

Plate LXXVII shows a portion of the Arduriis spectrum 
thus photographed, in comparison with spot and solar spectra. 
Barring some exceptions, which require further study, it will 
be noticed that the spectrum resembles the spot spectrum 
more closely than it does the solar spectrum.^ On account 

1 In comparing these spectra, changes of intensity should be noted with refer nee 
to adjoining (unaffected) lines in the same spectrum. Unavoidable differences of 
absolute intensity in the photographs prevent a satisfactory comparison, unless this 
precaution be observed. 



170 Stellae Evolution 

of the' imperfections of the Arcturus photograph, many of 
the lines are shown with less contrast than they would ex- 
exhibit in a really good negative. However, the illustration 
should be sufficient to indicate the important bearing of spot 
spectra on the question of stellar temperatures. 

The earliest classification of stellar spectra was that of 
Secchi, who distinguished four principal types: I, spectra 
of white and bluish-white stars, like Sirius, which contain 
broad and strong hydrogen and calcium lines, and but few 
lines, narrow and comparatively faint, of other elements; II, 
spectra of yellowish-white stars, like the Sun; III, spectra 
of red stars, containing a very characteristic series of bands, 
not identified by Secchi; IV, spectra of another class of red 
stars, containing the strongly marked bands of carbon. The 
bands in the spectra of stars of Secchi' s third type were 
finally identified by Fowler, who showed that they are due 
to titanium oxide. In view of the presence of these same 
bands in spot spectra (Fig. 2, Plate LXXIV), it becomes 
interesting to inquire whether the lines in stellar spectra of 
this type also resemble those in sun-spots. 

The brilliant red star Betelgeuze (a Orionis) which pre- 
sents so striking a contrast with the bluish star Rigel, in 
the constellation of Orion, is a good representative of the 
third type. It was accordingly selected to test the question. 
A dense flint glass prism belonging to the 5-foot spectrohelio- 
graph was substituted for the grating in the large stellar 
spectrograph of the Snow telescope, and the thermostat was 
modified so as to control the temperature of the air surround- 
ing the prism. In this way the spectrum of a Orionis was 
photographed by Adams, with a total exposure of seven hours 
on two consecutive nights. The work was done during the 
rainy season, and clouds, followed by continuous bad weather, 
cut short the exposure on the second night, and prevented the 
observations from being continued. The plate, while not 



Stellar Temperatures 171 

strong enough to be of the best quality, is nevertheless 
sufficiently good to serve for the purpose of a general com- 
parison. It was found that essentially all of the lines are 
stronger than in the Sun, and that lines which are strength- 
ened in spots are much more decidedly strengthened in a 
Orionis than lines unaffected in spots. In fact, the relative 
strengthening is much more marked in the case of this star 
than in the spots themselves, probably indicating that its 
temperature is lower. As the titanium oxide flutings form a 
conspicuous feature of the spectrum of a Orionis, and are also 
present in sun-spots, the evidence appears to be practically 
complete. More detailed investigations will undoubtedly 
reveal various discrepancies, due to differences in chemical 
composition or physical condition. Nevertheless, it may be 
said, in general, that the resemblance between the spectra of 
sun-spots and those of third-type stars is so close as to indi- 
cate that the same cause is controlling the relative intensities 
of many lines in both instances. This cause, as the laboratory 
work indicates, is to be regarded as reduced temperature. 

Thus we have been led, through the study of certain 
phenomena of our typical star, the Sun, and through their 
interpretation by laboratory experiments, to the considera- 
tion of the general question of stellar temperatures. Let us 
now inquire whether other independent methods can be 
applied to determine these temperatures, dealing first with 
the possibility of measuring directly the heat radiation of 
stars. 

The early experiments of Huggins and Stone failed, for 
lack of suitable apparatus, to detect the exceedingly small 
degrees of heat which reach us from stellar sources. Even 
Boys was no more successful in 1888, though he concen- 
trated the stellar radiations on his newly invented radio- 
micrometer, which would show to'o"0"¥o"o" ^^ ^ degree rise of 
temperature. With the sensitiveness used, -js^q-oq of the 



172 Stellar Evolution 

heat received by his telescope mirror from the full Moon could 
be detected. Yet the brightest stars produced no certain 
effect. As the result of this work, Boys was convinced that 
no star sends us as much heat as would be received from a 
candle at a distance of 1.7 miles, if there were no atmos- 
pheric absorption. 

The subject of stellar heat was investigated by E. F. 
Nichols at the Yerkes Observatory, in 1898 and 1900. The 
radiometer employed as the heat-measuring apparatus con- 
sisted of two circular vanes of mica, each about one-twelfth 
of an inch in diameter, attached to the opposite ends of a 
delicate cross-arm of drawn glass, cemented to a whip of fine 
drawn glass about one and one-quarter inches long. To the 
lower end of this system a minute mirror, made by silvering 
a fragment of very thin microscope cover-glass, was attached, 
and the whole was suspended by a very fine quartz fiber in 
a vacuum chamber. This radiometer was mounted on a pier 
in the coelostat room of the Yerkes Observatory. A coelostat 
reflected the starlight to a 24-inch mirror of 8 feet focal 
length, which concentrated the stellar rays upon one of the 
vanes, after entering the radiometer case through a window 
of fluorite, which is very transparent to heat radiations. By 
observing a scale reflected in the small mirror attached to 
the radiometer suspension, the deflection of the vane, which 
indicated the heating effect of the stellar rays, could be 
measured. In this way it was found that Arcturus sends us 
about as much heat as would be received from a candle six 
miles away, if there were no absorption in the atmosphere. 
Vega has less than half the thermal intensity of Arcturus. 

The extraordinary sensitiveness of the apparatus employed 
may be illustrated by some observations of a candle 2,500 
feet from the observatory. Heat from this candle, when 
concentrated on the radiometer vane of the 24-inch mirror, 
gave a deflection of about sixty-two scale divisions. On one 



Stellae Temperatures 173 

occasion the assistant extinguished the candle and placed 
his head in front of it when the signal was given, instead of 
uncovering the flame. The deflection caused by the heat 
radiation of his face, at a distance of 2,500 feet, was twenty- 
five scale divisions! With no atmospheric absorption, the 
number of candles in a group at a distance of sixteen miles 
could be determined from the average of a series of meas- 
urements of their total heat radiation. 

As Archtrits and Vega appear about equally bright to the 
eye, the greater heat radiation of the former star indicates 
that it sends out a larger proportion of the long (red) waves. 
If neither star possessed an absorbing atmosphere, it might 
then be concluded that Arciurus is cooler than Vega, but so 
much larger in angular diameter, when seen from the Earth, 
as to be fully as bright as Vega, and to send us more than 
twice as much heat. However, since we know that the 
absorbing atmosphere of stars like Arcturus is much denser 
than that of stars like Vega, this conclusion would not 
hold. We are therefore not in a position to judge from 
these experiments as to the relative temperatures of these 
stars. 

Lockyer has recently endeavored to determine the relative 
temperatures of stars by comparing their spectra, when 
photographed under similar conditions, in order to learn 
which of two stars sends us the greater proportion of violet 
light. In accordance with a well-known law, the proportion 
of violet light emitted by a luminous body increases as the 
temperature rises. By measuring the position of maximum 
intensity in the spectrum of a star, it should thus be possible 
to determine its temperature. Unfortunately, however, as 
already remarked, no absolutely safe conclusions can be based 
upon a test of this kind. Stars with dense atmospheres must 
appear red in color, no matter what their temperature, as com- 
pared with stars whose atmospheres are much less dense. For 



174 Stellar Evolution 

we have here just such a condition of things as we observe in 
the setting Sun, which appears red simply because the violet 
rays are more highly absorbed by our atmosphere than the 
red rays. It seems to be true that the older and cooler stars 
have denser atmospheres than the younger and hotter ones. 
It is thus probable that the stars whose spectra contain the 
greater proportion of red light actually are cooler than those 
in which the violet light is relatively stronger. But the 
fact remains that we are not warranted in basing determina- 
tions of stellar temperatures on measurements which so 
obviously depend upon the effect of atmospheres of unknown 
density. We will return to this question of stellar tempera- 
tures in a further consideration of the classification of stars 
(chap. xx). 



CHAPTER XIX 

THE NEBULAR HYPOTHESIS 

In the preceding chapters we have seen how the study of 
stellar evolution depends primarily upon the most accurate 
knowledge we can obtain of the Sun, regarded as a typical 
star. We have also examined certain methods of observing 
solar, stellar and laboratory phenomena, and have taken 
advantage of the opportunity afforded by the peculiarities of 
Sun-spot spectra to illustrate the mutual dependence of these 
various means of research. In passing to certain of the more 
general considerations underlying our subject, we may now 
examine some of the principal hypotheses which have been 
offered to account for the development of solar and stellar 
systems. 

Passing over the important speculations of Kant, and the 
conclusions drawn by Herschel from his extensive observa- 
tions, we reach the nebular hypothesis of Laplace. This cele- 
brated explanation of the origin of the solar system has domi- 
nated the world's thought since the very date of its publication. 
The eminence of its author, and the unique value of his 
great work on celestial mechanics, led to the immediate 
acceptance of his ideas, even when advanced in speculative 
form and without the support of mathematical analysis. 
The greatest physicists and astronomers of the nineteenth 
century have given the weight of their approval to the 
nebular hypothesis, and all calculations as to the age of the 
Sun have been based upon it. When viewing it in the light 
of recent destructive criticism, we must not forget the value 
of Laplace's speculations in directing thought and in seek- 
ing to account, by a single generalization, for a host of 

175 



176 Stellae Evolution 

observed phenomena. Nor must we overlook his remark 
that the hypothesis was presented "with the distrust which 
should be inspired by everything that is not the result of 
observation or calculation." The widespread and favorable 
influence exerted by the hypothesis on the intellectual life 
of the nineteenth century cannot be destroyed by recent 
developments. In the same way, the beneficial effect of 
Darwin's work on organic evolution would remain, even if 
the hypothesis of natural selection were forced from its place 
by that of mutation. 

As the original statement of the nebular hypothesis is 
not easily accessible to every reader, it seems desirable to 
include here a free translation of Note VII, at the end of 
Laplace's Exposition du systems du monde. A few para- 
graphs, dealing with more technical details, are omitted, but 
all of the essential features are retained. 

In seeking to trace the cause of the original motions of the 
planetary systems, the following five phenomena, enumerated in 
the last chapter (of Laplace's book), are available: the motions of 
the planets in the same direction and nearly in the same plane; the 
motions of the satellites in the same direction as the planets; the 
motions of rotation of these different bodies and of the Sun in the 
same direction as their orbital motions, and in but slightly different 
planes ; the small eccentricity of the orbits of planets and satellites ; 
finally, the great eccentricity of comets' orbits, as though their 
inclination had been left to chance. 

So far as I am aware, Buff on is the only one who has endeavored, 
since the discovery of the true system of the world, to trace the 
origin of the planets and their satellites. He supposes that a 
comet, falling upon the Sun, drove from it a torrent of matter, 
which reunited at a distance in several globes, varying in size and 
in distance from the Sun; these globes, having become opaque and 
solid by cooling, are the planets and their satellites. 

Laplace then goes on to show that, although this hypoth- 
esis might account for the first of the five phenomena 



The Nebular Hypothesis 177 

mentioned above, the others could not be explained by 
it. In seeking to discern their true cause, he continues as 
follows : 

Whatever be its nature, since it has produced or directed the 
motions of the planets, it must have embraced all of these bodies, 
and, in view of the prodigious distances that separate them, it 
could only have been a fluid of immense extent. In order to give 
them a nearly circular motion about the Sun, in the same direction, 
the fluid must have surrounded this body like an atmosphere. The 
consideration of planetary motions thus leads us to think that, as 
the result of excessive heat, the solar system originally extended 
beyond the orbits of all the planets, and that it contracted by suc- 
cessive steps to its present limits. 

In the assumed primitive condition of the Sun, it resembled 
those nebulae which are shown by the telescope to be composed of 
a more or less brilliant nucleus, surrounded by nebulosity which, 
in condensing toward the surface of the nucleus, transforms it into 
a star. If, by analogy, we conceive of all the stars being formed 
in this manner, we may imagine their earlier nebular state, itself 
preceded by other states, in which the nebular matter was more 
and more diffuse, the nucleus being less and less luminous. By 
going back as far as possible, we thus arrive at a nebulosity so 
diffuse that its existence could hardly be suspected. 

Philosophical observers have long been impressed with the 
peculiar distribution of certain stars visible to the naked eye. 
Mitchel has remarked on the improbability that the stars of the 
Pleiades, for example, could have been compressed within the 
narrow limits which inclose them by mere chance, and he has 
hence concluded that this group of stars, and similar groups in the 
heavens, are the effects of an original cause or of a general law of 
nature. These groups are the necessary result of the condensation 
of nebulae having several nuclei; for it is evident that, if the nebu- 
lar matter w^ere continually attracted by these various nuclei, they 
would ultimately form a group of stars like that of the Pleiades. 
The condensation of nebulae having two nuclei will similarly form 
stars lying very close together, and revolving about one another, 
like the double stars whose motions have already been observed. 

But how has the solar atmosphere determined the motions of 
rotation and of revolution of the planets and satellites ? If these 



178 Stellae Evolution 

bodies had penetrated deeply into this atmosphere, its resistance 
would have caused them to fall upon the Sun. We may thus con- 
jecture that the planets were formed at its successive limits, by the 
condensation of zones of vapors which the Sun, in cooling, must 
have abandoned in the plane of its equator. 

Let us recall the results given in a preceding chapter. The 
atmosphere of the Sun could not have extended out indefinitely. 
Its limit was the point where the centrifugal force, due to its 
motion of rotation, balanced the attraction of gravitation. Now, as 
cooling contracted the atmosphere and condensed at the surface of 
the Sun the molecules lying near it, the motion of rotation acceler- 
ated. For, from the law of areas, the sum of the areas described 
by the radius vector of each molecule of the Sun and of its atmos- 
phere, when projected on the plane of its equator, being always 
the same, the rotation must be more rapid when these molecules 
approach the center of the Sun. The centrifugal force due to this 
motion thus becoming greater, the point where it equals the weight 
is nearer the Sun. If we then adopt the natural supposition that 
the atmosphere extended, at some period, to an extreme limit, it 
must have left behind, in cooling, the molecules situated at this 
limit and at the successive limits produced by the acceleration of 
the Sun's rotation. These abandoned molecules must have con- 
tinued to revolve around the Sun, since their centrifugal force was 
balanced by their weight. But since this equilibrium did not 
obtain in the case of the atmospheric molecules in higher latitudes, 
their weight caused them to approach the atmosphere as it con- 
densed, and they did not cease to belong to it until this motion 
brought them to the equator. 

Let us now consider the zones of vapor successively left behind. 
To all appearances these zones should form, by their condensation 
and the mutual attraction of their molecules, various concentric 
rings of vapor revolving around the Sun. The mutual friction of 
the molecules of each ring should have accelerated some and 
retarded others, until they had all acquired the same angular 
velocity. Thus the linear velocities of the molecules farthest from 
the center of the Sun must have been the greatest. The following 
cause would also contribute toward the production of this difference 
of velocity. The molecules farthest from the Sun, which, through 
the effects of cooling and condensation, came together to form the 
outer part of the ring, always described areas proportional to the 



The Nebular Hypothesis 170 

time, since the central force which controlled them was constantly 
directed toward the Sun. This constancy of areas requires that 
the velocity increase as the molecules move inward. It is evident 
that the same cause must have diminished the velocity of those 
molecules which moved outward to form the inner edge of the ring. 

If all the molecules of a ring of vapor continued to condense 
without separating, they would finally form a liquid or solid ring. 
But the uniformity which this formation demands in all parts of 
the ring, and in their rate of cooling, must have rendered this 
phenomenon extremely rare. Thus the solar system offers only a 
single example of it, that of the rings of Saturn. In almost all 
cases each ring of vapor must have broken into several masses 
which, having only slightly different velocities, continued to revolve 
at the same distance around the San. These masses must have 
assumed a spheroidal form, with a motion of rotation correspond- 
ing in direction with that of their revolution, since their inner 
molecules had smaller linear velocities than their outer molecules; 
they thus formed as many planets in a vaporous state. But if one 
of them had possessed sufficient power of attraction to bring all the 
others successively together about its own center, the vaporous ring 
would thus have been transformed into a single spheroidal mass of 
vapor, revolving about the Sun and rotating in a direction corre- 
sponding to that of its revolution. This latter case has been the 
most common one. Nevertheless, the solar system offers an 
example of the first case in the four minor planets which lie between 
Jupiter and Mars ; unless we suppose, in agreement with M. Olbers, 
that they originally formed a single planet broken up by a violent 
explosion into several parts having different velocities. 

Now, if we follow the changes which ultimate cooling must 
have produced in the vaporous planets whose formation we have 
just pictured, we shall witness the production, at the center of each, 
of a nucleus which continues to develop through the condensation 
of the atmosphere surrounding it. In this state the planet exactly 
resembles the Sun in its primitive nebular condition. Cooling 
must thus have produced, at the various limits of its atmosphere, 
phenomena similar to those we have described ; that is to say, rings 
and satellites revolving around its center in the direction of its 
motion of rotation, and turning in the same direction upon them- 
selves. The symmetrical distribution of Saturn's rings about its 
center and in the plane of its equator naturally results from this 



180 Stellae Evolution 

hypothesis, and would be inexpHcable without it. These rings 
seem to me ever-present proofs of the original extension of 
Saturn's atmosphere and of its successive retreats. Thus the 
singular phenomena of the slight eccentricity of the orbits of the 
planets and satellites, the small inclination of these orbits to the 
solar equator, the identity in direction of the motions of rotation 
and revolution of all these bodies with that of the solar rotation : 
flow from our hypothesis and give it great probability. 

If the solar system had been formed with perfect regularity, 
the orbits of the bodies which compose it would have been circles 
whose planes, like those of the various equators and rings, would 
have coincided with the plane of the solar equator. But it may be 
conceived that the endless varieties which must have existed in the 
temperature and density of the various parts of these great masses 
produced the eccentricity of their orbits and the deviation of their 
motions from the plane of this equator. 

In our hypothesis, comets are strangers to the planetary system. 
In considering them, as we have done, to be small nebulae wander- 
ing from system to system, and formed by the condensation of 
nebular matter distributed with such profusion throughout the 
universe, we perceive that, when they arrive in the region of 
space where the solar attraction is predominant, it forces them 
to describe elliptical and hyperbolic orbits. But their motions 
being equally possible in all directions, they must move indifferently 
in all directions and at all inclinations to the ecliptic ; which is in 
agreement with observation. Thus the condensation of nebular 
matter, by which we have just explained the motions of rotation 
and revolution of the planets and satellites in the same direction, 
and in planes differing but slightly, also explains why the motions 
of comets do not agree with this general law. 

Laplace, after discussing the great eccentricity of comets' 
orbits, as bearing on the nebular hypothesis, continues as 
follows : 

If certain comets entered the atmospheres of the Sun and 
planets during the formative period they must have fallen upon 
these bodies, after pursuing spiral paths. The result of their fall 
would be to cause the planes of the orbits and the equators of the 
planets to deviate from the solar equator. 



The Nebular Hypothesis 181 

If in the zones left behind by the solar atmosphere there were 
molecules too volatile to combine among themselves or with the 
planets, they must have continued to revolve about the Sim. They 
would thus give rise to such an appearance as that of the zodiacal 
light, without offering appreciable resistance to the various bodies 
of the planetary system, either because of their extreme rarity, or 
because their motion is very nearly the same as that of the planets 
which they encounter. 

A close examination of all the details of the solar system adds 
still further to the probability of our hypothesis. The original 
fluidity of the planets is clearly indicated by the flattening of their 
figure, in conformity with the laws of mutual attraction of their 
molecules ; furthermore, it is proved in the case of the Earth by 
the regular diminution of weight from the equator to the poles. 
This condition of original fluidity, to which we are led by astronomi- 
cal phenomena, should show itself in the phenomena of natural 
history. But, to perceive it there, it is necessary to take into 
account the immense variety of combinations formed by all ter- 
restrial substances mingled together in a state of vapor, when the 
reduction of temperature permitted their elements to unite among 
themselves. It is also necessary to consider the enormous changes 
that this fall of temperature must have brought about successively 
within the Earth and upon its surface, in all formations, in the 
constitution and the pressure of the atmosphere, in the ocean, and 
in the bodies which it held in solution. Finally, consideration 
should be given to violent disturbances, such as great volcanic 
eruptions, which must have modified, at various epochs, the regu- 
larity of these changes. Geology studied from- this point of view, 
which unites it to astronomy, will acquire precision and certainty 
in many particulars. 

Although the nebular hypothesis received almost universal 
acceptance, objections and difficulties were brought forward 
at various times during the nineteenth century. The criti- 
cisms of Babinet and Kirkwood were followed by the argu- 
ments of Faye, who concluded that the planets, if developed 
from the ring-system of Laplace, should rotate in the oppo- 
site direction. Laplace had assumed that the rings which 
were to form the planets revolved like solid bodies, their 



182 Stellar Evolution 

outer edge traveling faster than the inner one. This would 
have involved forward rotation of the planets, as now observed. 
But such a condition of things could not have occurred — the 
rings, split asunder by the forces acting upon them, must 
have followed Kepler's laws, which would require the inner 
edge to move the faster. The rings of Saturn, held up by 
Laplace as a striking illustration of his views, were shown 
by Maxwell in 1859 to be composed of small bodies like 
meteorites. This was the result of a mathematical demon- 
stration that the rings, if solid, would fly to pieces. It was 
confirmed by Keeler, in 1895, by one of the most beautiful 
applications of the spectroscope ever made. According to 
Doppler's principle, the position of a line in the spectrum 
of a moving body depends upon the velocity of the mo- 
tion. This is true, even when the light is reflected from 
the surface of the moving body, after being received from 
the Sun. If the inner edge of Saturn'' s ring is moving 
faster than the outer edge, the lines in the spectrum of the 
ring should be increasingly bent toward the violet (on the 
approaching side of the planet) or toward the red (on 
the receding side), in passing from the outer toward the 
inner edge. Keeler's photograph of Satumi^s spectrum 
shows this to be the case. Thus we have certain proof that 
Satnrn^s rings are made up of meteorites, each moving at the 
velocity a satellite would have at the same distance from the 
planet. 

In spite of these and kindred objections, the nebular 
hypothesis, at least in its general outlines, retained its com- 
manding position until subjected to a searching test insti- 
tuted by Chamberlin and Moulton. The principal arguments 
brought together in Moulton's paper, entitled "An Attempt 
to Test the Nebular Hypothesis by an Appeal to the Laws of 
Dynamics,"^ and in the discussion of the question in Volume 

1 Astrophysical Journal, Vol. XI (1900), p, 103. 



The Nebular Hypothesis 183 

II of Cliamberlin and Salisbury's Geology, are briefly sum- 
marized below. 

In the paper just referred to, Moulton defines the nebu- 
lar hypothesis in much more general terms than Laplace 
employed. In other words, in order to make the test as 
complete as possible, he assumes that the original nebula 
might consist of a gas or of a swarm of meteorites, since 
Darwin had proved mathematically that the properties of 
gases may be fulfilled in a meteoroidal swarm. Moulton's 
discussion, moreover, does not insist upon the assumption of 
a very high temperature, since the progress of knowledge 
has shown that the present heat of the Sun may be accounted 
for as a result of the contraction of a nebula originally at a 
low temperature. Finally, the breaking-up of the nebula is 
not limited to the abandonment of rings, but is considered 
to include possible division by some fission process, the 
separated portions having contracted to form the planets and 
satellites. 

The fact that the revolutions of certain satellites, such as 
those of Uranus and Neptune, are in a retrograde direction, 
while the planes of the orbits of the four satellites of Uranus 
are almost perpendicular to the plane of the planet's orbit, 
is an old argument against the nebular hypothesis. While 
the former difficulty could easily be overcome, the great 
inclination of the orbits of these satellites and that of Nep- 
tune seems to be directly opposed to Laplace's views. In the 
second place, the masses of the various planets, as well as the 
densities of the rings from which they are supposed to be 
formed, are shown to be entirely out of harmony with what 
the hypothesis would lead us to expect. Again, the inner 
satellite revolves about Mars in a period less than a third of 
the planet's rotation, while the hypothesis would require its 
velocity to be much less than that of the planet's surface. 
Darwin has shown that the friction of solar tides might have 



184 Stellak Evolution 

retarded the rotation of Mars, without affecting the satellite's 
motion. But Moulton points out that the inner edge of 
Saturii's ring completes a revolution in about half the time 
of Saturn^s rotation period. At this great distance from 
the Sun, the very small tides could not have retarded suf- 
ficiently the rotation period of Saturn, unless they have 
been operating several thousand times as long as the Martian 
tides. An attempt to ascribe the effect to the satellites of 
Saturn proves equally futile. 

Moulton next endeavors to answer the question whether 
the supposed initial conditions could have developed into the 
existing system. We know that the molecules of a gas are 
moving about at velocities which increase with the tempera- 
ture. Near the surface of the original Laplacian nebula the 
velocities of the molecules, in the case of such light elements 
as hydrogen, would be so great that the molecules would 
overcome the power of gravitation and be dispersed in space. 
It would, therefore, be difficult to account for the abundant 
supplies of this gas now observed on the Sun. A stronger 
objection is afforded through the application of these prin- 
ciples to the planets. It is easy to calculate, through the 
known velocities of gaseous molecules and the masses of the 
planets, the power of each planet to retain an atmosphere. 
It is also possible to determine with the spectroscope whether 
atmospheres exist on the planets. Working in this way, 
Moulton shows the improbability that the diffuse Earth-Moon 
ring, with its low power of attraction, could have held any of 
the atmospheric gases or water vapor, when such concentrated 
bodies as the Moon and Mercury are unable at the present 
time to hold atmospheres. 

As we know the masses of the Sun and planets, the 
average density of the original nebula, when it extended to 
the orbit of Neptune, can be approximately calculated. 
Moulton finds this to be about xriToiroiyoir^T ^^ ^^^^^ ^^ water. 



The Nebulae Hypothesis 185 

In this extraordinarily rare nebula, whether truly gaseous or 
meteoroidal, it is shown that matter would have been left 
behind continually and that the formation of separate rings 
would be impossible — a conclusion reached by Kirkwood in 
1869. Moulton thinks it equally certain that a large mass 
could not have been detached by any fission process. Fur- 
thermore, even if a ring had been formed, he shows it to be 
utterly improbable that its matter could have been drawn 
together into a planet. 

Some of the above conclusions may perhaps be open to 
question, but the final argument seems to be unanswerable. 
It is a well-known principle of dynamics that the moment of 
momentum of a system of bodies not under the action of 
external forces is constant. The moment of momentum is 
defined by the sum of the products of the masses of all the 
particles by their velocities and by their distances from the 
center of the system. This quantity should remain abso- 
lutely unchanged, whether the system be in the form of a 
nebula occupying the whole of Neptune's orbit, or a group 
of planets revolving around the Sun. Making his assump- 
tions in such a w^ay as to be most favorable to the nebular 
hypothesis, Moulton obtains the following results for the 
moment of momentum: 

When the nebula extended to Mars' orbit M=32.176 

When the nebula extended to Jupiter's orbit M=13.250 

When the nebula extended to the Earth's orbit M= 5.690 

W^hen the nebula extended to ilfercttr^/'s orbit M= 3.400 

In the system at present M= 0.151 

Thus, instead of remaining constant, the moment of 
momentum is shown to decrease rapidly and irregularly. 
In spite of the precautions taken to favor the nebular 
hypothesis as much as possible, the moment of momentum 
of the original system comes out 213 times that of the 
present solar system. 



186 Stellae Evolution 

The papers of Chamberlin and Moulton contain other 
serious criticisms based upon the study of the moment of 
momentum of the system, and raise various additional difficul- 
ties. Thus the attenuated state of the rock-forming substances 
of the Earth in the Earth-Moon ring would probably have 
resulted in their condensation into solid particles. Again, no 
nebulae closely resembling the annulated solar nebula have 
yet been discovered. Without going further into details, and 
without necessarily admitting the finality of all the above 
arguments, it can hardly be denied that Laplace's idea of the 
development of the solar system must be reconstructed or 
abandoned. It remains to be seen what can be substituted 
for it. Two attempts in this direction will be described in a 
later chapter. 



CHAPTER XX 

STELLAR DEVELOPMENT 

The nebular hypothesis, as outlined in the last chapter, 
presents a picture of the development of a planetary system 
like our own. In testing it, recourse may be had both to 
theoretical investigations and to observations of various 
kinds, particularly of nebulae, which may throw light on 
the earlier stages of the process of condensation. It must 
be remembered that planets comparable in size with the 
members of our solar system would be quite invisible at 
the distances of the stars. However, in the stady of stellar 
evolution we are concerned primarily with stars, rather than 
with the planets that may accompany them. It is neverthe- 
less evident that the two questions cannot be considered 
independently, since the details of the processes that result 
in the formation of planets must be of the highest impor- 
tance in researches on the development of the central suns 
of which they may have formed a part. 

Herschel, whose mind was always occupied with the prob- 
lem of the structure of the universe and the formation of its 
individual members, thought he perceived in the nebulae 
evidences of growth and development. He supposed that 
the cloud forms, of irregular structure, which extend over 
vast regions of the heavens, represent the earliest and most 
rudimentary condition of stellar life. Condensation toward 
a center, brought about by the action of gravity, would be 
shown in such a cloud by increased brightness. Latest in the 
line of nebular existence Herschel placed the planetary nebu- 
lae, in whose symmetrical forms he saw illustrated some such 
condition as Laplace postulated for the primitive solar system. 

187 



188 Stellak Evolution 

The mystery of the planetary nebulae still remains un- 
solved, but evidence is lacking that they represent a more 
advanced state than such irregular cloud masses as the 
Great Nebula in Orion. Indeed, it must be admitted that 
the accumulation of observations, principally through the 
aid of photography, has rendered the problem of nebular 
development more complex than it appeared to Sir William 
Herschel. Thousands of nebulae, entirely unknown to him, 
have been brought to our knowledge through improvements 
in telescope design and the aid of the sensitive plate. These 
range in character from immense luminous tracts, such as 
are shown, intermingled with stars, in photographs of the 
Milky Way, to the definite outlines and highly suggestive 
structure of the spiral nebulae. Of all objects in the heavens 
these latter most strongly suggest the operation of some 
process of development. But not a single object, of this type 
was known to Herschel, and even to this day their enormous 
distance from the Earth has prevented the detection of any 
changes in form, which might point to the explanation of 
their origin.^ 

If we follow Herschel, and consider the simplest case of 
nebular development, we may suppose that through loss of 
heat by radiation a portion of a nebulous mass begins to 
condense toward a center. Although still wholly gaseous, 
and showing few points of difference from an ordinary nebula, 
we may regard such an object as representing the first period 
in the life of a star. In the heart of the Orion nebula, Plate 
XXI, are four small stars, which constitute the well-known 
Trapezium. Situated as they are in this enormous mass 
of gas, it is not difficult to picture them as centers of con- 
densation, toward which the play of gravitational forces 
tends to concentrate the gases of the nebula. It might 
therefore be expected that stars in this early stage of growth 

1 See chap. xxi. 



Stellar Development 189 

would show, through the spectroscopic analysis of their light, 
some evidence of relationship with the surrounding nebula. 
Now, this is precisely what the spectroscope has demon- 
strated. Not only these stars, but many others in the con- 
stellation of Orion, are shown by the spectroscope to contain 
the same gases that constitute the nebula. Moreover, they 
also partake of its motion through space. Finally, Frost and 
Adams have demonstrated the interesting fact that some of 
these stars are actually moving in orbits about dark com- 
panions situated in the very heart of the nebula. Since the 
orbital velocities of the moving stars are very high, it thus 
seems probable that the matter which constitutes the Great 
Nebula in Orion is exceedingly tenuous, offering little resist- 
ance to motion within it. 

Other examples of direct relationship between stars and 
surrounding masses of nebulae might be mentioned, but 
this one will suffice for our present purpose. We must now 
consider what changes in color and in spectrum accompany 
the further development of the star as it continues to lose 
heat through radiation. 

Fraunhofer was the first, in the opening years of the 
nineteenth century, to observe the spectra of the stars. The 
simple method he employed, which consisted in placing a 
prism over the object-glass of a telescope, has since become, 
through the skill and energy of Pickering, a wonderfully 
effective agent for the wholesale study of stellar spectra. 
To Fraunhofer the differences he perceived when comparing 
the spectra of different stars were of no meaning, since 
the work of Kirchhoff had not yet been done. But 
the photographs made under Pickering's direction at the 
Harvard College Observatory now tell a remarkable story 
to the initiated. In making these photographs, a large 
prism is mounted in front of the object-glass of a 
(refracting) telescope, which is directed to a field of stars 



190 Stellae Evolution 

and made to follow its apparent motion by a driving-clock. 
Under these conditions, each star-image in the field of the 
telescope is drawn out into a spectrum, which falls upon a 
photographic plate at the focus. If the rate of the driving- 
clock were perfect, each of these spectra would be extremely 
narrow, and the "lines" which cross it might not be per- 
ceptible. To give the spectra the necessary width, the 
prism is set with its refracting edge parallel to the diurnal 
motion, so that the spectra would drift on the photographic 
plate, if the telescope were at rest, in a direction at right 
angles to their length. In making the photographs, the rate 
of the driving-clock is slightly altered, so that the drift of 
the spectra during the exposure is just sufficient to give them 
the desired width. Without this drift, each "line" would 
be merely a point in the spectrum. Plate LXXIX illustrates 
how admirably the spectra of the various stars in the field 
are recorded, and brings before us evidence of the spectral 
diversity which is supposed to characterize the different 
stages of stellar growth. 

As already indicated (chap, xviii), the spectra of stars 
increase in complexity as the cooling process continues. The 
gaseous nebulae contain a few bright lines in their spectra, 
the most conspicuous one of which belongs to a gas ("nebu- 
lum") not yet discovered on the Earth. The other nebular 
lines are due to hydrogen and helium. Those stars of the 
'^ Orion type" which appear to be earliest in order of devel- 
opment contain no lines except those of hydrogen and helium, 
which are faint and very broad and diffuse. As these gases 
are found in the gaseous nebulae, and as the relationship 
of these stars to surrounding nebulous matter is otherwise 
apparent, there is every reason to believe that they represent 
the earliest phase of stellar life. The stars of the Trapezium, 
which have already been mentioned as organically related to 
the Great Nebula of Orion, are of this type. ^'Oinon'''' stars 



Stellak Development 191 

wliicli appear to be somewhat further developed, show lines 
of magnesium, silicon, oxygen, and nitrogen, in addition to 
those of hydrogen and helium (Plate LXXX). 

Next in order of evolution appear to be the white, or 
bluish-white, stars like Sirius (Fig. 1, Plate LXXXI). The 
spectrum of Sirius is marked by broad and conspicuous 
hydrogen lines, associated with narrow and faint lines of 
iron, sodium, magnesium, etc. It has been shown by investi- 
gations of certain pairs of stars, in which the two components 
are in rapid rotation about their common center of gravity, 
that stars like Sirius are much less dense than the Sun, their 
specific gravity not exceeding that of water. This, of course, 
is exactly what would be expected in an early stage of transi- 
tion from a gaseous nebula to a highly condensed star. 

It is well known through mathematical demonstration 
that the condensation of such a mass of gas as that in which 
the Sun originated must involve the production of a vast 
amount of heat. Indeed, the present solar radiation may be 
accounted for by supposing that the Sun's diameter decreases 
about 400 feet in the course of a year. The seemingly 
paradoxical fact that a gaseous mass, through loss of heat by 
radiation, will actually grow hotter as long as it remains in a 
gaseous condition, was demonstrated by Lane in 1870. The 
point is that the heat produced by shrinkage is more than 
sufficient to compensate for the loss by radiation. Conse- 
quently, the shrinking mass grows hotter as long as it remains 
purely gaseous. The time finally comes, however, when its 
outer parts, which radiate freely into space and are not pro- 
tected from loss by outlying masses of heated matter, are 
cooled to the point of condensation. That is to say, certain 
metallic elements present in a state of vapor condense into 
clouds made up of minute liquid drops, thus resembling our 
terrestrial clouds, which are caused by the condensation of 
water vapor. 



192 Stellar Evolution 

When this point is reached, radiation must take place 
mainly from the surface of the star. This would result in 
very rapid surface cooling, were it not for convection cur- 
rents, which rise from the interior and supply the heat lost 
by radiation. In the Sun we have strong evidence of the 
existence of such currents, which are represented by the 
bright filaments that constitute the granulated surface 
(chap, xi), and by the minute flocculi illustrated in Plate 
XXXIX. The darker spaces (pores) between the granulations 
probably represent the cooler descending vapors. The denser 
vapors, which perhaps occupy these darker regions, apparently 
lie below the general photospheric level, for it has recently 
been found at Mount Wilson that the spectrum of the Sun's 
disk, at points very near the limb, differs decidedly from the 
spectrum at the center. The hazy wings, which may be seen 
on either side of many lines photographed at the Sun's cen- 
ter, and are still more conspicuous in sun-spots, are greatly 
reduced in intensity at the limb (Plate LXXXII), This 
would seem to indicate that at the center of the Sun we are 
looking down into the regions between the granulations, to a 
level where the vapor is dense enough to produce the winged 
lines. Near the edge of the Sun, on the contrary, we look 
across the tops of the bright filaments, and therefore fail to 
receive light from the denser vapors below. The absorption 
of the higher and cooler vapors should produce a change in the 
relative intensities of the lines such as takes place in sun- 
spots (p, 159), but in much smaller degree. Observation 
shows this to be the case, but there is by no means a strict 
parallel between the two classes of phenomena, and judg- 
ment must be reserved for the present. One of the next 
steps will be to photograph the spectrum of a pore, if so 
minute an object can be separately observed. The inves- 
tigation, when completely worked out, should furnish a 
searching criterion as to the validity of the hypothesis of 



Stellak Development 193 

reduced temperature in spots, and as to the cause of certain 
phenomena in the Sun and stars. 

We have already seen (p. 173) that increased density of 
the absorbing atmosphere tends to reduce the proportion of 
violet and ultra-violet rays, and thus to introduce a yellowish 
or reddish tinge into the star's light. Such stars as Sirius 
do not possess dense absorbing atmospheres, and because 
of this fact and of their extremely high temperature, their 
spectra extend far into the ultra-violet. 

In passing from these white stars to the yellowish stars, 
which constitute the solar class, the continued process of 
condensation is accompanied by the production of an absorb- 
ing atmosphere similar to that of the Sun. Beginning in 
the ultra-violet, the absorption becomes more and more 
appreciable as the solar type of star is approached. The 
decrease of intensity, while most marked in the ultra-violet 
region, is also manifest in the blue and violet part of the 
spectrum, whereas the red, yellow, and green are not greatly 
affected. The natural result is a change of color, through 
a deficiency in blue light. For this reason, stars of the 
solar class are yellowish in hue. Langley has pointed 
out that the Sun would appear bluish-white, if its absorbing 
atmosphere were removed. Accompanying this change of 
color we have the decreasing strength of the hydrogen lines, 
and the increasing strength of the metallic lines, which 
become very numerous in Procyon (Plate LXXXI) and still 
more so in the Sun. 

As already remarked, this gradual increase of atmospheric 
absorption prevents us from basing conclusions as to relative 
stellar temperatures on the position of the maximum of 
intensity in the spectrum. We may fall back, however, 
upon comparisons of the relative intensities of certain 
lines, just as was done in the study of sun-spots described 
in chap. xvii. This method of classifying stars according 



194 Stellar Evolution 

to their temperature was applied by Lockyer many years 
ago. He found that the "enhanced lines," which are 
brightest in the spark spectrum of a metal, exist alone in 
certain stars. In other words, the arc lines of the same 
metal, which are strong at the lower temperature of the arc, 
and feeble or absent in the spark, are so much reduced in 
intensity in these stars as to be entirely invisible. 

Lockyer's contention that these changes of relative inten- 
sity afford a mode of classifying stellar spectra on a temper- 
ature basis was denied by many spectroscopists, because of 
the possibility that such changes might be produced in stars 
by different electrical conditions rather than by differences 
of temperature. The results obtained in our laboratory 
imitation of sun-spot phenomena, however, seem to favor 
the view that a temperature classification of stars, on the 
basis of the relative intensities of lines, is perfectly pos- 
sible. For in these experiments it was shown that 
when all electrical phenomena are excluded, a decrease in 
temperature of the radiating vapors is accompanied by an 
increase in intensity of the lines that are strengthened in 
sun-spots and in red stars. Since the spark lines are weak- 
ened under the same conditions, and since conclusive evi- 
dence of comparatively low temperature is afforded by the 
presence in these spectra of flutings due to substances which 
are broken up at the higher temperature of the Sun, the 
temperature hypothesis may perhaps be taken as affording a 
simple basis of classification. This statement is not made 
without some reservations, however, as indicated by the 
remarks at the end of this chapter, and by the discussion 
of Lockyer's meteoritic hypothesis in chap. xxi. Moreover, 
since this classification takes no account of the possible effect 
of mass and environment on spectral type, it is hardly likely 
to prove adequate. 

Let us now consider the phenomena of declining stars. 



Stellar Development 195 

which have passed beyond the solar stage and are fading into 
invisibility. It will be remembered that these stars are 
orange or red in color and that they may be divided into two 
classes, similar in appearance to the eye, but easily dis- 
tinguishable with the aid of the spectroscope. The first of 
these classes (Secchi's third type) includes certain bright 
stars, such as the red Antares, which is a conspicuous 
object in the southern heavens during the summer months. 
The second class (Secchi's fourth type) has no brilliant 
representative. Indeed, the brightest stars of this char- 
acter are but barely discernible by the naked eye, while the 
great majority are to be observed only with the aid of a 
telescope. 

In the spectroscope both classes show a spectrum vastly 
more complicated than that of stars in an earlier stage of 
growth. The broad lines of hydrogen, which are greatly 
reduced in intensity in the Sun, are still further reduced in 
the red stars. In fact, the dark hydrogen lines have in cer- 
tain red stars given place to bright lines, especially in the 
case of variable stars, whose light undergoes regular or 
irregular fluctuations. The most characteristic feature of 
the red stars, however, is the presence in their spectra of 
dark bands or flutings. In third-type stars the sharp edges 
of these bands lie toward the violet, while on the red side 
the intensity gradually decreases. These bands have been 
found by Fowler to be due to the oxide of titanium, which may 
be broken up at the higher temperature of the Sun, but exists 
in the spectra of sun-spots (Plate LXXIV) . The bands of the 
red stars of Secchi's fourth type face in the opposite direction, 
with their sharply defined boundaries toward the red. These 
bands, as Plates LXXXIII and LXXXIV illustrate, are due 
to carbon and cyanogen. Some of them are faintly present 
in the Sun, but in the fourth-type stars they are much more 
strongly developed. 



196 Stellae Evolution 

The extensive investigations of Yogel and Dun6r, made 
visually, have given lis much information regarding the 
spectra of the red stars. However, the fourth-type stars are 
so faint that only the bands in their spectra could be seen 
with the telescopes used by these investigators, and their 
numerous dark lines were beyond observation. The great 
light-grasping power of the Yerkes telescope rendered a 
photographic study of these spectra possible, with the results 
shown in Plates LXXXIII, LXXXIV, and LXXXV. When 
the fourth-type stars are ranged in a series, the gradual 
change of spectrum from star to star is well illustrated (Fig. 2, 
Plate LXXXIII). The carbon flutings become stronger and 
stronger, until in a star like 152 Schjellerup they are so dense 
that they cut out a considerable portion of the light. In the 
Sun, the Yerkes telescope shows the existence of a very thin 
layer of carbon vapor, lying in close contact with the photo- 
sphere. In the fourth-type stars we may suppose that the 
further process of condensation results in an increased 
development of carbon vapor, the absorption of which becomes 
the characteristic feature of the spectrum. 

Another important point brought out by this investigation 
is the close relationship existing between the line spectra of 
third- and fourth-type stars. As will be seen by an exami- 
nation of Plate LXXXY, the line spectra of fi Geminorum 
and 74 Schjellerup seem to be almost precisely identical in 
certain regions. The presence of titanium oxide bands in 
the one case, and the carbon flutings in the other, compli- 
cate the comparison of the line spectra in other regions, 
though much is yet to be learned on this subject through 
further study of these stars with spectrographs of the highest 
dispersion. 

In view of the resemblance of the line spectra, it is diffi- 
cult to understand the diversity of the band spectra in the 
two great classes of red stars. Among third-type stars all 



Stellar Development 197 

intermediate types of spectra may be found between the Sun 
and the most advanced representative of the class. It might 
thus seem, especially in view of the close relationship 
between sun-spot and third-type spectra, that the cooling of 
the Sun would result in the formation of a third-type star. 
However, although no such perfect continuity has been shown 
to exist in the transition from solar to fourth-type stars, it 
seems possible that stars intermediate in character between 
280 Schjellerup (see Plate LXXXV) and the Sun may yet be 
discovered. Should this prove to be the case, and a more 
rigorous test be found to confirm the observed resemblance 
of the line spectra of the third and fourth types, the ques- 
tion whether a star like the Sun will develop into a third- or 
into a fourth-type star would be difficult to answer. This 
problem, which is one of the most interesting of those con- 
nected with the study of stellar evolution, will occupy a 
prominent place in the working programme of the Solar 
Observatory. 

Although the red stars represent the last period of lumi- 
nous stellar life, there remain to be considered the dark stars, 
which have been discovered in spite of their complete invisi- 
bility. Hundreds of these objects are already known to us 
through spectroscopic observations. They are members of 
double or triple systems, moving in orbits about a common 
center of gravity. Their existence has been inferred from 
measurements of the oscillation of the spectral lines, which 
move back and forth toward the red or toward the violet, as 
the star under observation recedes from and then approaches 
the earth in its orbital motion. Obviously, it is the spectrum 
of the visible star which can be observed, but motion in an 
orbit necessarily implies the existence of a companion star, 
which may or may not be luminous. If sufficiently bright 
to be visible, it may not be separated from its close neighbor 
in the most powerful telescopes. But in the spectroscope 



198 Stellae Evolution 

the lines of the composite spectrum will appear double, twice 
in each orbital revolution of the pair. If only one star is 
bright enough to give a spectrum, its lines will simply oscil- 
late to and fro. 

Hitherto we have tacitly assumed, in harmony with cur- 
rent views, that all stars are built on a single model, and that 
each passes through the same stages of development in its 
transition from the nebular condition to the solid state. It 
should be pointed out here, however, that many circumstances 
warn us against implicit acceptance of such a law of uni- 
formity. The assumption that a given type of spectrum 
represents a given stage of growth involves the idea that the 
chemical composition of all stars is essentially the same, and 
that the particular position of the star in the universe, and 
other conditions which may obtain in individual cases, are 
matters of no importance. While it is true that we have 
strong reasons for belief in the universal distribution of most 
of the chemical elements known on the Earth, and the uni- 
versal operation of the law of gravitation, and of all other 
laws which define terrestrial conditions, the assumption that 
identically the same course is pursued by every star in 
passing from its origin to its final decay is entirely un- 
warranted. We must be prepared to meet widely diverse 
conditions and to observe modifications in the process of 
development which are determined directly by such con- 
ditions. 

Take the case of the Pleiades, for example (Plate 
LXXXVI). Here we have a group of stars entangled in 
nebulosity, and moving together through the heavens. 
Every indication goes to show that this is an organic group, 
whose members are of common origin. But the spectra of 
practically all of these stars, irrespective of size and bright- 
ness, are of Secchi's first type. How are we to believe that 
widely different masses will pass through their evolutional 



Stellar Development 199 

steps with equal rapidity? An appeal to double stars, whose 
members are undoubtedly of common origin, does not help 
matters. We invariably find that the fainter member of the 
system, which might have been supposed to cool most rapidly 
because of its smaller size, is yellow or red in color, while its 
larger companion is more nearly white, or tinged with blue. 
Huggins argues, however, that the greater surface gravity 
possessed by stars of large mass may cause more rapid change 
in spectral type. Thus a large star of low density may be no 
farther advanced in spectral type than a smaller but more 
highly condensed star. According to Huggins' views, the 
early steps in evolution would be characterized by small 
gravity at the surface, comparatively slow changes of tempera- 
ture in passing oat ward from the interior, and convection 
currents less violent than those observed in the Sun. If the 
star were hot enough, hydrogen might be the only gas suf- 
ficiently cool, with respect to the radiation from below, to 
show itself by absorption lines. Vapors of greater density 
would lie lower in the star, where their temperature might 
be so nearly that of the region behind them that their lines 
would not appear in the spectrum. 

Schuster, who has done an important service in empha- 
sizing the elements of weakness in the assumed law of uni- 
formity, nevertheless believes that most of the spectral types 
represent stages in the development of stars. Thus, while 
he does not maintain that all stars pass through an identical 
series of changes, he agrees with the view that the general 
course of development lies along similar lines, though impor- 
tant modifications may enter in particular cases. The order 
of development which he favors is as follows : 

(1) Helium or Orion stars. 

(2) Hydrogen or Sir Ian stars. 

(3) Calcium or Procyon stars. 

(4) Solar or Capellan stars. 



200 Stellar Evolution 

In describing the process of condensation, Schuster points 
out that the expansion caused by the rising temperature of 
the gaseous bodies must at first result in the rejection of 
helium, hydrogen, and other light gases, on the supposition 
that the gravitation is not sufficient to retain them. These 
light gases will thus be left to constitute diffuse nebulous 
masses, as illustrated by the gaseous nebulae, particularly 
by the nebulous regions in such a group as the Pleiades. 
In the process of time, however, the star will have condensed 
sufficiently to retain hydrogen and helium, and these gases 
will then begin to diffuse into the interior, where they will 
be absorbed at a rate which depends upon the star's mass. 
Helium, which is denser than hydrogen, will be retained 
first, thus giving rise to the helium, or Orion, stars. As this 
gas diffuses inward, its place will be taken by hydrogen, 
which will thus become predominant in the spectrum. In 
its turn, the hydrogen will diffuse into the star, and the 
increasing convection currents will cause a more and more 
complete stirring-up of the low-lying metallic vapors, which 
will therefore play an increasingly prominent part in the 
spectrum. Thus the solar stage will ultimately be reached. 

An interesting point in this explanation is the consider- 
able possibility of variation which stars of different mass and 
in different environments may exhibit. If but little hydrogen 
happens to be in the neighborhood, the process of conden- 
sation may not result in the attraction of a sufficient 
quantity of this gas to produce the hydrogen type of spec- 
trum. Again, the star may be of such low density that it is 
unable to attract hydrogen, and thus it may pass into the 
solar stage without exhibiting strong hydrogen lines. There 
may also be stars of such small mass that, in spite of having 
condensed sufficiently to attract hydrogen, they are not able 
to absorb it all, and therefore they may continue to exhibit 
a spectrum of the first type without ever passing into the 



Stellae Development 201 

solar stage. Furthermore, in the case of two stars of equal 
age but different mass, the larger may have passed to the 
condition of Arciurus (incipient red star), while the other is 
still in the solar stage, because of its more rapid absorption 
of hydrogen. 

This theory gives an interesting explanation of the above- 
mentioned spectral phenomena of double stars, since it indi- 
cates that the larger star, through its power of absorbing 
hydrogen more rapidly and completely, may pass to the solar 
stage, while the smaller one continues to give a spectrum of 
the first type. Schuster agrees with Huggins that the small 
mass would lose heat more rapidly than the larger one, but 
believes that the type of spectrum may be more completely 
controlled by the rapidity with which the hydrogen is 
absorbed. 

It is evident that the highly suggestive views of Schuster 
should stimulate much research. The distribution of spectra 
of different types through the heavens is a subject of great 
interest, and doubtless has an important bearing on the 
question of stellar evolution. Certain types of stars, for 
example, tend to cluster thickly in the Milky Way, while 
others show no such tendency. Pickering's work in photo- 
graphing the spectra of an immense number of stars in the 
northern and southern heavens offers most valuable material 
for the study of this subject. The investigation may perhaps 
be extended to fainter stars with the 60-inch Mount Wilson 
reflector, through the use of a spectrograph having no slit 
and so designed as to record photographically the spectra of 
all stars lying within a certain field. But since this field 
can hardly exceed 20' of arc in diameter, it would not be 
feasible to photograph the entire heavens in this way.^ 

However, a most important scheme of co-operation has 
been instituted by Kapteyn, for the purpose of obtaining 

1 The objective prism photographs cover a field several degrees in diameter. 



202 Stellar Evolution 

data bearing upon the problem of the geometrical stucture 
of the universe and the distribution of stars within it. 
Through the impracticability of securing all necessary data 
for stars distributed over the entire heavens, Kapteyn has 
selected certain limited areas of the sky, so distributed as to 
render it probable that conclusions based upon a complete 
study of the stars within these areas will be likely to apply 
to the heavens at large. The application of the 60-inch 
reflector to the photography of stellar spectra by the above- 
mentioned process will therefore be confined, for the most 
part, to Kapteyn's areas, where many other observers are 
already gathering information, in accordance with a plan 
which allots to each institution the work for which its instru- 
ments are best adapted. For Kapteyn's purposes, only the 
general type of spectrum is required, since he is primarily 
concerned with questions of distribution and structure, rather 
than those which relate to the evolution of stars. The data he 
desires include determinations of the brightness, distance, 
and motions of the stars within the selected areas. The 60- 
inch reflector, on account of its great light-gathering power, 
can assist materially in those portions of this work which 
relate to the faintest stars. The investigation of the motions 
of these stars in the line of sight is necessary from the evo- 
lutional standpoint, because community of motion may mean 
organic relationship of stars in a group, as in the case of 
the Pleiades. Photometric investigations and the study of 
parallaxes are also required, since, when the distance and 
brightness of a star are known, its mass can be determined, 
if certain reasonable assumptions as to the surface brilliancy 
are made. We have just seen how important a factor the 
mass of a star may be in determining the course of its 
evolution. 

Enough has been said to indicate the nature of the work 
which large telescopes may perform. The direct photography 



Stellak Development 203 

of nebulae may provide the means of detecting, in the course 
of years, changes in their form bearing directly upon the 
manner and rate of their condensation. The photographic 
study of their spectra may help to explain why a few nebulae 
show the bright line spectra of gases, while the very numerous 
spiral nebulae appear to have merely a continuous spectrum. 
With the high dispersion of powerful spectrographs like the one 
shown in the constant-temperature chamber in Plate XCVI, 
the spectra of a few of the brightest stars in the heavens, 
which include most of the spectral types, can be minutely 
analyzed. In this way, and with the aid of smaller spectro- 
graphs, the spectra of red stars of the third and fourth types 
can be examined to much better advantage than previously, 
with reference to their relationship to the Sun, to sun-spots, 
and to one another. It is evident that these investigations, 
with others on the spectra of stars of various classes, the 
distribution of the different types of spectra within Kapteyn's 
selected areas, and studies of the brightness and parallaxes 
of the same stars, might well involve the co-operative use of 
several telescopes of the largest size. 



CHAPTER XXI 
THE METEORITIC AND PLANETESIMAL HYPOTHESES 

In even the briefest outline of the methods of studying 
stellar evolution, reference must be made to two hypotheses 
which are intended by their authors to take the place of the 
nebular hypothesis of Laplace. In both of these, swarms 
of meteorites, rather than matter in the gaseous state, are 
supposed to afford the raw material of which stellar systems 
are compounded. The nature of the swarms, however, is 
unlike in the two cases. According to Lockyer, the meteorites 
are to be regarded as analogous to the wandering mole- 
cules of gases, in that they move indiscriminately in all direc- 
tions and at widely different velocities. Sir George Darwin 
has, indeed, demonstrated mathematically that a meteoritic 
swarm, constituted in this way, is closely analogous to a 
gas. The meteorites move rapidly about, colliding with one 
another from time to time, just as the molecules of a gas are 
supposed to do, according to the kinetic theory. Chamberlin 
and Moulton, on the contrary, assume their meteorites to be 
revolving in well-defined orbits, and therefore suffering only 
such collisions as may result from certain meteorites over- 
taking others of lower velocity.^ 

The most characteristic nebular line is a brilliant one in 
the green part of the spectrum, attributed to an unknown 
gas, which has been called "nebulum." According to 
Lockyer, this line is the remnant of a complicated fluting 
in the spectrum of magnesium oxide, with the brightest 
part of which he found it exactly to coincide. In his view 

1 In his explanation of globular and spiral nebulae, and of certain other celestial 
pheijomena, Lockyer also assumes the meteorites to revolve in well-defined orbits. 

204 



Meteoritic and Planetesimal Hypotheses 205 

the rest of the fluting is invisible only because of the faint- 
ness of the nebular line. This has been completely dis- 
proved, however, by Keeler's remarkably precise measures 
of the chief nebular line at the Lick Observatory. His 
observations show, not only that the chief nebular line does 
not correspond in position with the head of the magnesium 
fluting, but also that it differs entirely from it in appearance. 
It is therefore not possible to regard magnesium oxide as a 
constituent of the nebulae. The green line may with far 
greater probability be considered to represent a very light 
gas, not yet discovered on the Earth. 

Lockyer's conclusions as to the origin of the chief nebular 
line play an important part in his meteoritic hypothesis. 
He believes that the frequent collisions between meteorites 
in the swarms produce sufficient heat to volatilize certain 
constituents of the meteorites, which are rendered luminous, 
so that their lines should appear in the nebular spectrum. 
Lockyer tried the experiment of heating fragments of mete- 
orites in a tube, from which the air had been partially 
exhausted. He found that hydrogen, hydrocarbon vapors, 
and the vapor of magnesium oxide were given off from the 
meteorites. When an electric discharge was passed through 
the gases in the tube at reduced pressure, the spectrum was 
found to consist of the lines of hydrogen, the characteristic 
flutings given by compounds of carbon, and the green fluting 
of magnesium oxide, to which reference has been made. In 
comets, which are known to be intimately associated with 
meteorites, the flutings due to compounds of carbon form 
the most characteristic feature of the spectrum. But although 
certain astronomers have believed these flutings to be present 
in the spectrum of the nebulae, their conclusions are not 
confirmed by the majority of observers, who can neither 
see nor photograph any trace of the flutings. The only 
remaining connection between the nebulae and the gases 



206 Stellar Evolution 

derived by Lockyer from meteorites therefore depends upon 
the presence of hydrogen in both cases. But hydrogen is 
so universally distributed among the celestial bodies that its 
absence from nebulae would almost be regarded as an anom- 
aly requiring explanation. It therefore cannot be said that 
much weight is to be accorded to the experimental basis of 
the meteoritic hypothesis. 

It ought to be said, in favor of the hypothesis, that it 
provides a simple way of accounting for the existence in 
the nebulae of substances not represented in their spectra, 
but which appear in stars evolved from nebulae. If a 
nebula is to be regarded as a glowing gas, in which all 
substances contained in stars exist in a state of vapor, 
it remains to be shown why a very few gases manifest 
their presence by the appearance of their bright lines in 
the spectrum, whereas all the other elements produce no 
lines, and therefore give no indication of their existence. 
In this connection it must not be forgotten that in mix- 
tures of various vapors the spectra of some of the vapors 
appear when an electric discharge is passed through the 
mixture, while the lines due to certain other vapors remain 
invisible. Too little has been done, however, in this im- 
portant field of research, to permit final conclusions to be 
drawn. For this reason no one is at present able to say in 
what form the iron, nickel, and other metals, which sub- 
sequently make their appearance in the stars, can exist in 
the nebulae. 

This question is, indeed, but one of the many mysteries 
which at present surround the nebulae (Plates LXXXVI- 
XC). We have no knowledge, for example, why they glow 
with a steady and unchanging light, since there is no direct 
evidence that this light is produced either by heat or by 
electrical excitation. It must not be forgotten that very few 
nebulae are certainly known to be gaseous: thousands of 



Meteoritic and Planetesimal Hypotheses 207 

them seem to give a continuous spectrum, in which the bright 
lines of gases do not appear. Whether this is due to the 
presence of solid or liquid matter, to pressure effects, or to 
other causes, is not yet known. The process by which stars 
are condensed out of nebulae is also not clearly understood. 
It cannot depend wholly upon some action connected with the 
spiral form, since, as already stated, we have in the Orion 
nebula, which is not a spiral, one of the best-known examples 
of direct relationship between stars and nebulae. It is now 
rather commonly believed that, while the temperature of 
small particles in the nebulae may be very high, the mean 
temperature of the entire mass may nevertheless be very low, 
since it has been pointed out by Huggins that the appear- 
ance presented by the nebulae could be produced by widely 
separated luminous particles. In view of all these facts, it 
may therefore be said that much work remains to be done 
on the nebulae, not only in photographing their forms, but in 
investigating their spectra, and in interpreting them through 
laboratory experiments. 

Starting from the meteoritic hypothesis, and assuming 
that the chemical elements, at the temperature of the hottest 
stars, are dissociated into simpler substances, Lockyer has 
developed a plan of stellar evolution which comprises a classi- 
fication of stellar spectra on a temperature basis. He sup- 
poses that the meteoritic swarms represented by the nebulae 
gradually condense into stars, by processes whose details are 
still uncertain. According to his classification, the gaseous 
and bright-line stars, in which the temperature is supposed 
to be higher than that of the less condensed nebulae, lie just 
above the latter in point of development. Then come the 
red stars of Secchi's third type : though it may appear to many 
spectroscopists that the difficulty of tracing a connection 
between their spectra and those of the stars placed just before 
them would be altogether insuperable. Further conden- 



208 Stellak Evolution 

sation, still involving a rise of temperature, would produce 
stars analogous to the Sun, but differing in the important 
particular that, while their temperature is increasing, that 
of the Sun is supposed to be decreasing. Finally, at the 
point of maximum temperature, Lockyer places stars of 
Secchi's first type. Here the meteorites, long since com- 
pletely transformed into the gaseous state, have reached the 
condition implied by Lane's law, at which the rise in super- 
ficial temperature, due to continued condensation, is just 
balanced by the loss resulting from radiation. The declining 
period, then setting in, results in the development of stars 
like the Sun, which can be only arbitrarily distinguished from 
stars of equal, but rising, temperature, lying on the opposite 
branch of the temperature curve. After the solar stars come 
the red stars of Secchi's fourth type, and after these, final 
extinction of light. 

This system of classification, considered apart from the 
hypotheses with which it is connected, has the advantage of 
providing for both the ascending and descending branches 
of the temperature curve. Unfortunately, we are perhaps not 
yet in a position to distinguish clearly between stars of the 
same surface temperature, in one of which the gain of heat 
is more rapid than the loss, while in the other the reverse 
is true. As already remarked, the assumption that the red 
stars of Secchi's third type lie not far above the nebulae is also 
a difficult one to admit. But the classification nevertheless 
deserves careful consideration, and the most searching tests 
that can be applied. 

As the late Miss Gierke has well said, the complex struc- 
ture of meteorites suggests a highly developed, rather than an 
elementary, condition of existence. This, however, is hardly 
to be taken as an objection to Lockyer's hypothesis, since 
the manner in which the meteoritic swarms came into exist- 
ence is not postulated. The planetesimal hypothesis, however, 



Meteoritio and Planetesimal Hypotheses 209 

begins with a Mly organized sun, which is supposed, in its 
motion through space, to come into the immediate neighbor- 
hood of another sun, equal to or greater than itself. The 
effect of the attraction between the two bodies would be to 
reduce the immense restraining power of the Sun along the 
line of mutual attraction, i. e., in the direction of the other 
sun, and in the opposite direction. Under certain conditions 
the Sun is observed to shoot out prominences with velocities 
approaching 300 miles per second. If the velocity exceeded 
382 miles per second, the matter projected from the Sun 
would escape the power of its attraction and move off into 
space, never to return. If another great body were passing 
near the Sun, the tendency toward eruptions would be greatly 
augmented along the line joining the two bodies, and immense 
protuberances would doubtless be projected at high velocities 
from opposite ends of the solar diameter corresponding 
with this line. 

According to the planetesimal hypothesis, the two pro- 
tuberances would be formed as the two suns were swinging 
past one another around their common center of gravity. 
The effect of mutual attraction would be to cause the two 
great arms to assume a spiral form, in which the scattered 
materials revolve about the central mass in elliptical orbits. 
Moulton has shown, by rigorous mathematical tests, that just 
such a result might actually occur, and that the forms of 
the spiral nebulae may thus be closely imitated (Plates 
LXXXVIII-XC). Although the matter shot out from the 
Sun would necessarily be gaseous, the hypothesis assumes 
that it would rapidly cool down to a finely divided solid con- 
dition.^ The outer portions of the protuberances would 
naturally be formed from the surface materials of the Sun, 
while the inner extremities would come mainly from lower 

1 How, it may be asked, can these small bodies remain brilliantly luminous for 
many years? And why do we not discover incipient spirals, giving a bright line 
spectrum? 



210 Stellae Evolution 

depths, where the heavier elements are found. This may 
possibly explain the lightness of the outer planets of our solar 
system, and the great relative weight of the inner ones. The 
changing attraction of the neighboring star might also cause 
a series of irregular outbursts, accounting for the knotty and 
uneven distribution of the matter in the spirals (Plate XC). 
Chamberlin points out that a very small fraction of the Sun's 
mass, not exceeding 1 or 2 per cent., would be amply sufficient 
to supply all of the matter required to form a planetary 
system like our own. 

In the further evolution of the system, the central mass 
is supposed to form the sun, the knots to serve as the nuclei 
about which the planetary materials gather, and the remain- 
ing diffuse nebulous matter to be swept up by the nuclei or 
absorbed by the sun. The building-up of the planets is not 
supposed to take place, as in the nebular hypothesis, simply 
through the gravitational attraction of the planetary nuclei 
on the matter surrounding them. On the contrary, the main 
agency is assumed to be a gradual accretion of the mass 
through collisions of isolated planetesimals (meteorites) 
resulting from the intersection of the individual orbits, 
brought about periodically through the rotation of their line 
of apsides. Thus it is held, according to this hypothesis, 
that the Earth was never a molten mass, but that it was built 
up by gradual accretions. Chamberlin was led to this view 
of the condition of the Earth's interior from various geo- 
logical considerations, which seem to him inconsistent with 
the hypothesis of a fluid origin. 

If this book were a treatise on stellar evolution, all of 
these questions would require much fuller discussion and 
criticism, and space would necessarily be devoted to the 
remarkable phenomena of variable and temporary stars, the 
tidal investigations of Darwin and their possible bearing on 
the evolution of double-star systems, and many other subjects 



Meteokitic and Planetesimal Hypotheses '211 

which have not received consideration. Enough has been 
said, however, to give an idea of the nature of the problems 
which an observer concerned with stellar evolution is called 
upon to attack, and the general character of some of the 
observational methods required to solve them. 



CHAPTER XXII 

DOES THE SOLAR HEAT VARY ? 

One does not often stop to think of the delicate balance 
that determines the conditions of life on the Earth. But it 
is obvious enough that a small change in the intensity of the 
solar radiation would suffice to transform the climate of the 
temperate zones to that of the equatorial or polar regions. 
A greater change might soon result in the complete destruc- 
tion of life. 

It is therefore a matter of the most vital interest to 
inquire into the source and constancy of the Sun's heat. 
What fuel maintains the great fire that warms and lights us, 
and supplies, through its beneficent influence on growing 
crops, the food that we consume? Is the average daily 
influx of solar rays constant and unchangeable, and are we 
justified in our tacit belief in the inexhaustibility of the 
supply? Such thoughts, seriously pondered by students of 
solar physics, have led to extensive investigations, which 
must go on for many years before these questions can be 
finally answered. 

As we have already seen, the contraction of a nebulous 
mass to form a star, or a sun like our own, must result in 
the liberation of much heat. Indeed, the total solar radia- 
tion in the course of a year can be accounted for on the sup- 
position that the Sun's diameter decreases about 250 feet in 
this time. Since the discovery of radium, which possesses 
the remarkable property of sending out heat, with little evi- 
dence of exhaustion, for very long periods of time, it has 
been suggested that this substance, if it exists in the Sun, 
may be the source of part of its radiation. Radium has not 

212 



Does the Solak Heat Vaky? 213 

yet been detected in the Sun with the spectroscope, but it 
may lie at low levels, where its vapor would take no part in 
the absorption that produces the lines of the solar spectrum. 
The abundance of helium in the Sun suggests that radium, 
which gives off this gas during the disintegration process, 
may perhaps exist within or beneath the photosphere. 

If radium really supplies any considerable part of the 
Sun's heat, its ultimate exhaustion would involve a decided 
decrease in the solar radiation. As we are not yet certain, 
however, that there is any radium in the Sun, the possibility 
of such a contingency may be regarded as too remote for 
profitable speculation. 

We may take it for granted that the Sun will continue to 
radiate heat, at practically the present average rate, for many 
centuries to come. But do we know that the rate is abso- 
lutely constant? May not fluctuations occur of sufficient 
magnitude to affect our climate appreciably, and to be 
reflected in the ebb and flow of crops and the price of wheat ? 

Until a short time ago this question had been tested in 
only the roughest way. It was known that sun-spots pass 
through a regular cycle of change, occupying about eleven 
years. A curve was accordingly drawn, showing the varying 
number of sun-spots, and compared with a curve represent- 
ing, for example, the varying price of wheat. As the two 
were thought to show some correspondence in form, it was 
held that the price of wheat is determined by the solar 
activity, as measured by the number of spots. 

But the correspondence of the two curves was far from 
perfect, and might have resulted from mere chance. Rain- 
fall and temperature curves have given results that appear 
more satisfactory, but the whole question is still in its primi- 
tive stages, and little that is absolutely definite and reliable 
has been learned. The efforts now being made by the Solar 
Commission of the International Meteorological Committee 



214 Stellar Evolution 

may be expected to help matters, but much will depend upon 
the appliances used to measure the solar radiation, and to 
determine the amount of heat lost by absorption in the 
Earth's atmosphere. 

The most elaborate study of this question yet made is 
due to the late Secretary Langley, of the Smithsonian Insti- 
tution. He long ago recognized that the chief difficulty of 
the problem lies in the constantly varying absorption of the 
air above us. If measures of the solar radiation could be 
made from a point outside of our atmosphere, any observed 
fluctuations would be due to the Sun itself. But near the 
level of the sea the difficulties are very great. 

To diminish them, Langley led an expedition to Mount 
Whitney in California. Here, at an elevation of over 15,000 
feet, the denser and more variable half of the atmosphere is 
left below. The precision of the measures was thus greatly 
increased, but the expedition was not able to remain long 
enough to determine whether the so-called "solar constant" 
of radiation is actually a constant, or undergoes changes of 
an irregular or a periodic character. 

Langley strongly felt the importance of continuing this 
work with the greatly improved apparatus developed by 
Abbot and others at the Smithsonian Astrophysical Obser- 
vatory in Washington. He therefore recommended that the 
Carnegie Institution make provision for further researches 
of this nature at a mountain station. When the Solar Ob- 
servatory was established, a co-operative arrangement with 
the Smithsonian Institution was accordingly entered into, 
and measures of the solar constant were made daily by Abbot 
on Mount Wilson during the summers of 1905 and 1906. 

The apparatus used in this work is most ingenious. Two 
independent operations are carried on simultaneously: the 
direct measurement of the solar radiation with some form of 
pyrheliometer ; and the determination of the atmospheric 



Does the Solar Heat Vary? 215 

absorption, for all the colors of the spectrum, with a bolometer 
(Plate XCI). 

The pyrheliometer, in the form used by Abbot, measures 
the rise in temperature, in a given time, of a known volume 
of liquid exposed to the Sun's rays. If there were no atmos- 
phere, pyrheliometer measures alone would suffice to furnish 
the desired information. But the heat of the Sun at noon 
is far greater than shortly after sunrise, since the rays pass 
through a much shorter air-path. Consequently, the obser- 
vations must be repeated at regular intervals throughout the 
morning. 

The bolometer, invented by Langley, is so sensitive 
to radiation that it will measure a rise in temperature of 
less than one-millionth of a degree. It consists of two 
very fine threads of platinum, about g-y^T ii^ch thick, 
mounted side by side within a constant temperature chamber. 
One of these is shielded, the other exposed to the radiation 
to be measured. The platinum threads form two of the arms 
of a "Wheatstone's bridge," and are connected wdth a stor- 
age battery, so that a feeble current constantly passes through 
them, A galvanometer of the most sensitive type is so bal- 
anced in the circuit that its reading is zero when the currents 
flowing through the two platinum threads are equal. The 
moment the resistance of the exposed strip is changed by 
radiation falling upon it, the galvanometer is deflected by 
an amount which measures the heating effect of the radiation. 

In practice, the solar spectrum is caused to move slowly 
across the exposed bolometer thread. The galvanometer 
needle then swings back and forth, giving small deflections 
when a dark line or absorption band is passing over the 
bolometer, and large deflections when the full intensity of 
the spectrum is being measured. To record the motions of 
the needle a minute mirror, attached to it, is caused to reflect 
a spot of light upon a photographic plate. The same mechan- 



216 Stellar Evolution 

ism that moves the spectrum across the bolometer causes 
this plate to travel slowly downward. Thus the deflections 
of the needle are photographically registered upon the plate. 
With the aid of such curves the total atmospheric absorption, 
measured separately for each region of the spectrum, is 
accurately determined. The reduced pyrheliometer readings, 
corrected in this way for absorption, give the value of the 
solar constant. 

With such highly developed instruments the systematic 
study of the solar radiation was pursued in Washington. On 
the best days, which came none too often, the refinement of 
the method permitted the atmospheric absorption to be elimi- 
nated, even at this station so near the level of the sea. It 
was soon found that the values of the "solar constant" were 
not constant, but variable. Indeed, differences as great as 
10 per cent, of the whole were encountered. Was it safe to 
conclude that the solar radiation undergoes variations of this 
considerable amount? 

On Mount Wilson the escape from the denser air of the 
valley, the purity of the upper sky, and the constant succes- 
sion of perfectly clear days, permitted the question to be 
put to the test. Day after day the Sun was followed through 
the heavens, from a time soon after it rose above the eastern 
mountains to its culmination near the zenith. Sometimes 
the work was continued through the afternoon, but the morn- 
ing observations proved to be sufficient. 

As soon as the curves had been measured and reduced, 
and the pyrheliometer observations plotted, the full advan- 
tages of the mountain station appeared. Not only was the 
precision of the work much greater than before: even more 
important was the fact that daily observations, continued for 
many weeks, brought the exact nature of the phenomenon to 
light. Through the latter part of the month of July, 1905, 
the value of the solar constant increased slightly from day 



Does the Solak Heat Vaey? 217 

to day, until it readied a maximum. It then declined in the 
same gradual manner. From these results Abbot concluded 
that the solar heat had temporarily undergone actual change, 
not to be ascribed to any modification of our own atmosphere. 

Does this mean a greater outpouring of the solar radia- 
tion, caused by an actual increase in the surface temperature 
of the Sun? Or had the absorption of the solar atmosphere 
decreased for a time, returning later to its normal value? 
Much study will be required to answer this question, though 
the uncertainties may be partially cleared up when the 1906 
observations have been reduced. Increased solar activity, 
represented by numerous sun-spots and flocculi, may prob- 
ably be taken to indicate the existence of more numer- 
ous and more violent convection currents, bringing larger 
quantities of heat from the Sun's interior to the surface. 
At times of great solar activity, therefore, we might expect 
increased radiation. But this might soon be checked by 
the diffusion through the solar atmosphere of materials 
thrown upward by the violent eruptions, which characterize 
such periods of activity. Indeed, the increased absorption, 
persisting after the subsidence of unusual activity, might 
result in a reduction of the radiation below its normal value. 

Evidently a comparison must be made between observa- 
tions of various kinds, carried on simultaneously. Spectro- 
heliograph plates, bearing the record of the area covered by 
the flocculi, afford an index to the solar activity. The 
absorption of the solar atmosphere may also be measured by 
allowing the solar image to drift slowly across a bolometer, 
and photographing the galvanometer deflections upon a fall- 
ing plate. During the summer of 1906 both of these classes 
of work were carried on at Mount Wilson, simultaneously 
with Abbot's measurements of the solar constant. When all 
the results are discussed together, new light may be thrown 
on the subject. 



218 Stellae Evolution 

But the work is barely started, and must be continued for 
many years under the best conditions. Simultaneous obser- 
vations at several widely separated mountain stations are 
greatly to be desired, to make certain that local changes in 
our own atmosphere are in no wise concerned in the apparent 
solar changes. Moreover, the work should go on without 
the interruptions caused by the rainy season. If, for example, 
a bolographic outfit were established at the Solar Observatory 
at Kodaikanal, in south India, at an elevation of 7,000 feet, 
the dry season there would correspond with the rainy season 
in southern California. An Australian station might also 
accomplish very important results. It is to be hoped that 
adequate provision may soon be made to carry out this im- 
portant work. 

But, it may be asked, must not such fluctuations of the 
solar radiation, if real, be the cause of marked changes of 
terrestrial temperature, easily detected and obvious in their 
effects? Abbot believes that the thermometric records do 
actually reflect these solar variations, but Newcomb holds the 
contrary view. It is evident that complex meteorological 
phenomena may be involved, and that their disentanglement 
may require long-continued research. For this reason the 
studies of the solar radiation undertaken by the Interna- 
tional Union for Co-operation in Solar Research, the co- 
operation in meteorological work set on foot by the Solar 
Commission, and the labors of such an institution as the 
observatory recently established on Mount Weather, Vir- 
ginia, by the United States Weather Bureau, should prove 
of value. In the exhaustive study of so important a problem 
the cordial co-operation of many investigators is essential to 
success. 



CHAPTER XXIII 

THE CONSTRUCTION OP A LARGE REFLECTING 
TELESCOPE 

The grinding and polishing of a 60-inch mirror involve 
a variety of operations, described in detail in Ritchey's 
memoir On the Modern Reflecting Telescope and the 
Making and Testing of Optical Mirroi^s,^ the most authori- 
tative treatise on the subject. A brief account of these 
operations, taken in large part from the above source, may 
be of interest here. 

It is first necessary to obtain a suitable disk of glass. 
The disk (of plate glass) made by the French Plate Glass 
Works, of St. Gobain, France, for the reflecting telescope of 
the Solar Observatory is 60 inches in diameter, 8 inches 
thick, and weighs a ton. It must be remembered that the 
requirements for a large mirror are very different from those 
for a lens through which light is to pass. The mirror disk 
is merely a support for the thin silver film on its front sur- 
face, from which the light is reflected without entering the 
glass. For this reason the great perfection of a lens disk is 
not necessary. Nevertheless, the glass must be free from 
striae and other evidences of irregularity of structure. It 
should contain no large bubbles, though a few small ones, if 
they do not lie on the surface, are not objectionable. The 
most important condition, however, is freedom from strain 
caused by imperfect annealing. Evidences of strain are 
detected by a test with polarized light. Such a test, how- 
ever, cannot be final, as an incident in the history of a great 
telescope objective illustrates. The disk had been carefully 

1 Published by the Smithsonian Institution. 

219 



220 Stellar Evolution 

annealed and was supposed to be suitable for its purpose. 
During the process of grinding it flew to pieces, on account 
of internal strain, the serious nature of which had not been 
recognized in the test with polarized light. 

It may not be obvious why the disk must be so thick, 
when its sole purpose is to support the thin film of silver 
on its accurately figured face. Great thickness, however, 
is absolutely essential, to diminish the effects of bending 
due to the weight of the glass and to temperature changes. 
The thickness of a mirror should not be less than one- 
eighth or one-seventh of the diameter. Even with such 
thickness a special support system is necessary to prevent 
flexure. 

Glass is chosen in preference to other materials for tele- 
scope mirrors because of its uniformity of structure, com- 
parative ease of working, and capacity for a high polish. 
Its lightness, when compared with such substances as specu- 
lum metal (formerly employed for telescope mirrors), is 
an important advantage. Furthermore, a surface of pure 
silver, first used by Foucault, refiects a much larger propor- 
tion of light than polished speculum metal. 

The grinding-machine, designed and constructed by 
Ritchey for his work on the 60-inch mirror, is shown in 
Plate XCII. The glass disk rests on a heavy cast-iron turn- 
table, carried by a vertical steel shaft. Between the lower 
surface of the glass (ground flat) and the turn-table are two 
thicknesses of Brussels carpet, which form an admirable 
support during the grinding and polishing process. The 
edge of the glass is ground true by means of a rapidly 
rotating iron face-plate, held against the disk while the turn- 
table is slowly rotated. The cutting material is powdered 
carborundum, carried down between the glass and the face- 
plate by a slow stream of water. After the edge-grinding 
is completed, the two faces of the glass are ground plane 



A LakCtE Reflecting Telescope 221 

and parallel, before the process of making one of these sur- 
faces concave is undertaken. 

The grinding-tools employed for this work are circular 
plates of cast-iron, strongly ribbed on the back, and divided 
into a series of small squares on the grinding surface, by 
two sets of parallel grooves, planed at right angles to one 
another. The tool rests on the surface of the glass, though 
in Plate XCIII it is shown suspended from the lever arm, 
employed to swing the heavy tools into or out of position. 
During the grinding the disk is slowly rotated and the tool, 
also kept in rotation, is moved over its surface in a series 
of strokes from four to eight inches in length, by means 
of the arm shown above the disk in Plate XCIII. On its 
right-hand extremity this arm terminates in a steel shaft, 
which moves back and forth through a swiveled bearing 
supported on an adjustable slide. In this way the position 
of the grinding-tool on the disk can be changed laterally, 
so as to bring the stroke across the center of the glass or 
near the edge. If it is found, for example, that the center 
is being cut away too rapidly, the tool is moved near the 
edge and the grinding continued there until the error is 
corrected. The tool is not kept at any one position for a 
great length of time, to avoid producing low zones in the 
glass. 

For the grinding process, various grades of carborundum 
are prepared in the following way: The powdered carbo- 
rundum is mixed with water and thoroughly stirred. After 
settling for two minutes the coarse particles reach the bottom 
of the bucket and the liquid, containing "two-minute" car- 
borundum and the finer grades, is siphoned off into another 
bucket. After the contents of the second bucket have been 
allowed to stand four minutes, the liquid is poured off and 
the "two-minute" carborundum at the bottom of the bucket is 
set aside for fine grinding purposes. In the same way, carbo- 



222 Stellar Evolution 

rundum which has remained in suspension for periods up to 
one hundred and twenty minutes, or even longer, is prepared, 
These very fine grinding materials are used to give the 
smooth and almost polished surface obtained after the grind- 
ing with coarser carborundum is completed. 

A perfectly true Brown & Sharpe steel straight-edge is 
used to determine whether the surface of the glass is approx- 
imately plane. When it is found to be sufficiently so for 
the preliminary work, the fine grinding is commenced,* 
beginning with two-minute carborundum and continuing 
with finer grades. In this work the iron grinding-tool is 
counter-poised by placing weights on a lever arm con- 
nected by a shaft with the tool. The pressure is reduced 
from one-third pound to the square inch for the five- or ten- 
minute carborundum, to about one-twelfth pound per square 
inch for the one-hundred-and-twenty- and two-hundred-and- 
forty-minute carborundum. Unless this precaution is taken 
there is great danger of scratching the glass. 

After being fine ground, the back of the mirror is 
polished with rouge in the manner described later. No 
great pains are taken with this surface, although it is made 
very nearly plane, and is then polished so as to permit silver- 
ing (Plate XCIV). It is desirable to silver the back of the 
mirror, as well as the front, in order to prevent temperature 
changes from affecting the two surfaces in unequal degree. 

The front surface, after it has been given a plane figure, 
is ready to be made concave. For this purpose a convex 
iron tool, of suitable curvature, is employed. In the case of 
the 60-inch mirror the radius of curvature is 50 feet. The 
curvature of the tool, and also of the glass, is tested from 
time to time by a spherometer. This consists of a tripod, 
with a micrometer screw at its center, which permits the 
deviation of the surface from a plane to be accurately deter- 
mined. After the desired curvature has been secured, the 



A Large Reflecting Telescope 223 

fine grinding is carried to a point where the surface is very 
smooth and ready for polishing. 

The polishing and figuring are done by means of a tool 
built up of narrow strips of wood, saturated with paraffine to 
prevent change of figure. The face of this tool is covered 
with squares of rosin, of a certain degree of hardness, which 
can be determined only by experience. The rosin squares 
are finally coated with a thin layer of beeswax, which forms 
the polishing surface. The soft wax is very useful, since 
small hard particles that may happen to be present in the 
polishing material are likely to bed themselves in it, thus 
reducing the danger of scratches. As a preliminary to 
polishing, the tool is placed in contact with the glass disk 
and pressed against it, by weights placed on the back, so 
that it may acquire the same curvature as the surface. 
After pressing for some hours, until the waxed squares 
appear smooth and bright in all parts, the polishing may 
begin. This is accomplished by moving the tool over the 
rotating glass, by the main arm of the machine, as in the 
case of the grinding process. The polishing material is 
powdered jewelers' rouge, used commercially for polishing 
plate glass. The fine rouge is separated from impurities 
and coarser particles by a washing process similar to that 
used for carborundum. The rouge, mixed with distilled 
water, is applied to the surface of the glass by means of a 
wide brush of cheese-cloth. 

The greatest precautions must be taken throughout the 
polishing process to avoid scratches. For this purpose the 
room in which the work is done is fitted up in such a way as 
to eliminate danger from dust. In the polishing-rooms of 
the Solar Observatory optical shop (Plate XCV) the plas- 
tered walls and ceilings are heavily varnished, and a canvas 
screen is hung above the glass, to protect it from any falling 
particles. The cement floor is painted, and kept wet when the 



224 Stellar Evolution 

polishing is in progress. The windows are double and care- 
fully sealed, outer air being admitted to the room through a 
cheese-cloth filter. The temperature is maintained constant, 
within two or three degrees, by means of a hot-water furnace, 
controlled by a thermostat. The motor, driving-shaft, and 
apparatus for varying the speed of the grinding-machine, 
are carefully inclosed, only the slow-moving belt coming out 
into the room. No one is permitted to enter the room except 
the optician, who wears a surgeon's gown and cap. By 
observing such precautions the work may be continued for 
months without producing even microscopic scratches in the 
glass surface. 

We may now assume that the glass has been polished, 
after receiving an approximately spherical surface. It then 
becomes necessary to apply a more accurate test than the 
spherometer permits. For this purpose the glass is turned 
into a nearly vertical position, where it is supported by a steel 
edge-band (Plate XCV). An artificial star, consisting of 
a hole about 3^ J-g- of an inch in diameter illuminated by an 
acetylene lamp or other brilliant source of light, is placed at 
the center of curvature, 50 feet from the glass surface. The 
light from the articifial star then falls upon the disk and is 
reflected back so as to form an image close beside the pin- 
hole. If the surface is perfectly spherical, it will appear, 
when examined by the eye placed at this point, to be bril- 
liantly and uniformly illuminated. With an eye-piece, the 
image of the pin-hole will then be perfectly sharp, showing 
the most minute details or irregularities of the hole itself. 

It is much more probable, however, that the surface will 
have many zonal errors. To detect and interpret these, the 
"knife-edge test," due to Foucault, is employed. If all the 
zones come to a focus at the same point, and a knife edge is 
moved across this point, while looking at the glass, the light 
will be cut off instantly from all parts of the disk. If, how- 



A Large Reflecting Telescope 225 

ever, the curvature of certain zones is greater or less than 
the average curvature, these zones will resemble projecting 
or receding rings on an otherwise uniformly bright surface. 
The effect is as though the light were shining from one side, 
producing an appearance of relief by lights and shadows. 
The test is so sensitive that an error of -s-oijVo"^ P^^^ ^^ ^^ 
inch can be detected. If, for example, the finger is placed 
for a few moments on the glass, the heating of the surface 
will cause a swelling easily to be detected by the knife-edge 
test. 

The process of figuring consists in removing the high 
and low zones by means of the polishing tool, the stroke 
and position of which must be modified in accordance with 
the results of the knife-edge test. After a perfectly spheri- 
cal form has been obtained in this way, the difficult process 
of changing the spherical to a paraboloidal surface is begun. 
As is well known, the parallel rays from a star, falling on a 
spherical surface, will not be brought to a focus at a central 
point, but in an irregular figure, called a "caustic." A 
paraboloid, however, brings all parallel rays to a single 
focus, and produces a perfect stellar image. In the case of 
the 60-inch mirror, which has a focal length of 25 feet, the 
paraboloid is deeper than the sphere at the center of the 
disk by a quantity less than j^^-^o^ ^^ ^^^ inch. Months of 
figuring are required, however, to produce this small differ- 
ence, because of the necessity of giving each zone of the 
paraboloid precisely the right curvature. In testing the 
surface from the center of curvature, the measured radius of 
each narrow zone of the mirror (the other parts being 
covered by a cardboard screen) must correspond with the 
calculated radius. The extreme difficulty of accomplishing 
this may be appreciated when it is remembered that the 
deviation of any zone from the surface of a perfect para- 
boloid must not be greater than tu o^fuQ-o" ^^ ^^ inch, which 



226 Stellae Evolution 

would correspond to a change of y^-q ^^ ^^ i^^h in the 
radius of curvature. 

When parallel light is available, the difficulties of secur- 
ing a perfectly satisfactory test of a paraboloidal mirror are 
greatly reduced. In this case the mirror, when seen from 
its focal plane (25 feet from the glass, or one-half the 
radius of curvature) appears like a uniformly illuminated 
plane surface when a perfectly paraboloidal form has been 
obtained. This method of testing with parallel light has 
been developed by Eitchey, and was used by him to secure 
the last degree of perfection in the figure of the 60-inch 
mirror. 

As already explained, the problem of mounting a large 
mirror is quite as serious as that of figuring it. It is neces- 
sary, in the first place, to support the mirror in such a way 
that it will retain its form, without bending, in any position 
of the telescope. Furthermore, it must be held so that it 
will not slip laterally, since the slightest change in the posi- 
tion of the mirror with respect to the tube will cause a dis- 
placement of the star images on the photographic plate. 
The mirror, thus supported, must be carried at the lower 
end of a tube, of skeleton construction, open at the top, and 
so mounted that it can be pointed toward any part of the 
heavens and made to follow the apparent motion of the 
stars by rotation about an axis parallel to the axis of the 
Earth. Strength and stability of the mounting, freedom 
from flexure, perfection of optical and mechanical construc- 
tion and adjustment, and the greatest precision of driving — 
all these conditions must be met before a large reflector can 
be expected to give satisfactory results, in the more exacting 
departments of photographic work. 

The difficulties thus presented have been most successfully 
solved by Ritchey, whose design for the mounting of the 
60-inch mirror is shown in Plate XCVI. The telescope tube 



A Lakge Reflecting Telescope 227 

is hung between the arms of a massive cast-iron fork, which 
is bolted to the upper end of the polar axis. This axis, a 
hollow forging of nickel steel, is inclined at an angle corre- 
sponding to the latitude of Mount Wilson (34° 13') and 
thus rendered parallel to the axis of the Earth. Leveling 
screws, by which the base of the mounting is supported on 
its pier, permit this adjustment to be made with great pre- 
cision. In order to relieve the great friction of this axis on 
the upper and lower bearings in which it lies, a hollow steel 
float, 10 feet in diameter, is bolted to its upper end, just 
below the fork. This float dips into a tank filled with mer- 
cury. Thus the entire instrument is floated by the mercury, 
and in this way the friction on the bearings is reduced to a 
minimum. 

The 60-inch mirror rests at the lower end of the tube, on 
a support system consisting of a large number of weighted 
levers, which press against the back of the glass and dis- 
tribute the load. A similar series of weighted levers around 
the circumference of the mirror provide the edge support. 
The path of the rays from the star may be as shown in 
Plate XCVII, Figs. 1, 2, 3, or 4. In the first arrangement 
(the Newtonian telescope), the parallel rays, after striking 
the mirror, are reflected back and would come to a focus at a 
point just beyond the end of the tube. They are intercepted, 
however, by a plane mirror of silvered glass, which turns 
them at right angles and forms the image on the photo- 
graphic plate, which is mounted on the side of the tube near 
the upper end. In this case the focal length of the instru- 
ment is 25 feet, and the image is formed without secondary 
magnification. 

If, however, it is desired to secure, for certain classes of 
work, the advantages of a greater focal length, a different 
arrangement is adopted. The upper section of the tube, 
bearing the plane mirror, is removed, and a shorter section 



228 Stellar Evolution 

substituted for it. This carries a hyperboloidal mirror, 
which returns the rays toward the center of the large mirror 
and causes them to converge less rapidly. They then meet 
a small plane mirror, supported at the middle of the tube 
near its lower end, which sends them to one of the following 
instruments, mounted in the focal plane: (1) a double-slide 
plate-holder, carrying a sensitive plate, for the photography 
of the Moon, planets, bright nebulae, etc., with an equivalent 
focal length of 100 feet (Fig. 3) ; (2) a spectrograph mounted 
in place of this photographic plate, in which case a convex 
mirror of different curvature is employed, and the equivalent 
focal length is 80 feet (Fig. 4) ; or finally (3) a third convex 
mirror may be used and the plane mirror inclined so as to 
form the star image (after sending the light down through 
the hollow polar axis) on the slit of a powerful spectrograph, 
of 13 feet focal length, mounted on a pier in a constant- 
temperature chamber (Fig. 2). In this case the equivalent 
focal length is 150 feet. 

The telescope is moved in right ascension or declination 
by electric motors, controlled from the floor of the observing- 
room. The driving-clock moves the telescope in right ascen- 
sion by means of a worm-gear, 10 feet in diameter, carried 
by the polar axis. The cutting of the teeth of this worm- 
gear is a mechanical operation requiring the highest precision 
of workmanship. Each tooth was spaced off by means of a 
finely divided circle attached to the polar axis, and read with 
a microscope. The rotating cutter was driven by an electric 
motor. After all the teeth had been cut, the worm and worm- 
gear were ground together for many hours, until all slight 
residual errors had been eliminated. The operation was 
completed with jewelers' rouge, which leaves a smooth and 
highly polished surface. 

All of the heavy parts of this mounting were made, after 
Ritchey's designs, by the Union Iron Works Company, of 



A Large Reflecting Telescope 229 

San Francisco. They were then shipped to Pasadena, where 
the mounting has been erected in tlie Solar Observatory shop 
(Plate XCVIII). Here the worm-gear was cut, and all of 
the smaller parts, including the driving-clock, setting-circles, 
slow motions, motors, etc., are being fitted and adjusted. 
All of these parts were made in the Observatory instrument 
shop, which is equipped with the best machinery obtainable 
for work of this kind (Plate XCIX). 

As soon as this mounting has been completed, the 60-inch 
mirror will be put in place and the telescope thoroughly 
tested, by actual photography of the heavens. It will then 
be necessary to transport the instrument to Mount Wilson — 
an operation of considerable difficulty, as several of the cast- 
ings are very large, and weigh about five tons each. 

The building for the 60-inch reflector is of steel con- 
struction throughout (Plate C). The thin inner walls will 
be shielded from the Sun by outer walls, and air will be per- 
mitted to circulate in the space between the two. The dome, 
60 feet in diameter, will be rotated by an electric motor, 
either rapidly, when passing from one part of the heavens 
to another, or at a slow, uniform rate, of such a speed as 
to keep the opening (15 feet wide) constantly opposite the 
end of the telescope tube, when it is following a star. The 
observer, when photographing in the principal focus, will 
stand on a platform suspended from the dome and rotating 
with it. The double-slide plate-carrier, with which stars 
and nebulae will be photographed, is similar to that used 
with the Yerkes telescope (Plate XVII). 



CHAPTEK XXIV 

SOME POSSIBILITIES OF NEW INSTRUMENTS 

In looking toward the future and endeavoring to imagine 
what appliances will be employed by the astronomer of the 
next generation, the line of least resistance is to consider the 
possibilities of improving existing telescopes and the auxil- 
iary apparatus employed with them; for the prevision of 
more radical departures is beyond our province. It is safe 
to predict that the equatorial refractor, of which the Lick 
and Yerkes telescopes are types, will hold an important 
place in observatories for many years to come. The ease 
with which such instruments can be pointed toward any part 
of the heavens; the absence of reflecting surfaces; the per- 
manence of object-glasses, as contrasted with the necessity 
of silvering mirrors from time to time ; the convenient posi- 
tion of the observer at the lower end of the tube, rather than 
at the upper end of a Newtonian reflector: these and other 
considerations point to the long-continued use of the standard 
refractor. In its most perfect form this instrument is still 
capable of some improvements, the most important of which 
will be the introduction of truly achromatic object-glasses, 
capable of uniting the rays of all colors at the same focus. 

It seems probable that the uses of the equatorial refractor 
will be confined more and more to visual observations, and 
to certain departments of photography, especially those 
involving great precision of measurement or the inclusion 
of large fields on a single plate. For the latter work the 
refracting telescope, particularly in the portrait-lens form, 
possesses great advantages, on account of the very limited 
field of the reflector. It does not at present appear desir- 

230 



Some Possibilities of New Instruments 231 

able to increase the aperture of refractors beyond the limit 
of 40 inches reached in the Yerkes telescope. The resolving 
power of such an aperture, when the atmospheric condi- 
tions are good enough to permit its realization, is suffi- 
ciently great for the most exacting demands of visual work. 
Increased light-gathering power, which is much to be desired 
for the investigation of faint objects, will be most easily and 
effectively obtained through the use of large reflecting tele- 
scopes. Increased focal length, on the other hand, which is 
needed to give larger solar images, can best be secured 
through the use of some form of fixed telescope. We may 
now consider what types of telescopes are likely to prove 
most serviceable in photographic and spectroscopic studies 
of the Sun, stars, and nebulae. 

Many important investigations require the use of a tele- 
scope giving a sharply defined solar image, of large diameter, 
at a fixed position within a laboratory. The focal length of 
such a telescope must not change rapidly when the instru- 
ment is exposed to the Sun. The image must not rotate, 
and the laboratory conditions must permit the successful use 
of the largest and most powerful spectrographs and spec- 
troheliographs. The Snow telescope meets most of these 
requirements in a very satisfactory manner. The one diffi- 
culty with this instrument is the distortion of the image and 
the change of focus when the mirrors are exposed for some 
time to the Sun. When the precautions described in chap. 
XV are taken, these obstacles are easily overcome in cur- 
rent work with the 5 -foot spectroheliograph and the Littrow 
spectrograph. But with long exposures, such as are required 
with a spectroheliograph of very high dispersion, the change 
of focus during the exposure would be a serious obstacle. It 
is probable that by substituting very thick mirrors for those 
now used in the Snow telescope, and by reflecting sunlight 
upon their rear surfaces, which should be silvered like the 



232 Stellae Evolution 

front surfaces, the tendency to distortion could be overcome. 
For a very thick mirror would resist the bending which 
results from the expansion of the front surface; and even if 
the figure were changed, the compensating effect produced 
by heating the rear surface should restore it. But the Snow 
telescope is fully occupied with its present work, for which 
it is well adapted. Accordingly, a new type of fixed tele- 
scope has been devised for the purpose of supplementing the 
Snow telescope, particularly in photographic work involving 
long exposures. 

In the new instrument the coelostat, provided with mir- 
rors a foot thick, will be mounted at the summit of a steel 
tower 65 feet in height (Fig. 7). From the second mirror 
the sunlight will be sent vertically downward to a 12-inch 
object-glass, mounted a short distance below it. This object- 
glass, of 60 feet focal length, will form an image of the Sun 
near the ground level. The new instrument will thus consist 
essentially of a fixed refracting telescope, pointing directly 
to the zenith and receiving light from a coelostat and second 
mirror. 

The spectroscopic laboratory at the base of the tower will 
be excavated in the earth, to insure constancy of temperature 
and great stability of the instruments it will contain. Of 
these, the one shown on the left in Fig. 7^ is a Littrow 
spectrograph, similar to the one employed with the Snow 
telescope, but of much greater power. This instrument will 
have a focal length of 30 feet, and be provided with a large 
plane grating. On the right is shown a spectroheliograph of 
30 feet focal length, designed for extending the monochromatic 
photography of the Sun to many of the finer lines of the 
spectrum (p. 236). The atmospheric calm that prevails on 

1 This is only a general diagram, omitting all details, such as the steel house, at 
the base of the tower, which covers the upper ends of the spectroscope and spectro- 
lieliograph: tlie small electric elevator, to convey the observer from the bottom of 
the underground laboratory to the summit of the tower, etc. 



Some Possibilities of New Instruments 233 

Mount Wilson during the best observing season may permit 
the inner tower to be used merely as a skeleton, if firmly 
stayed in position by strong steel guy-ropes. If, on experi- 
ment, it is found that the wind produces too much vibration 
of the structure, an outer tower, covered with canvas louvers,^ 
will be erected to shield the inner one, as indicated in Fig. 7. 

It remains to be seen whether this type of telescope will 
meet the rigorous conditions demanded in the case of a fixed 
instrument for solar research. The vertical beam of light 
should be less affected by unequal temperature conditions 
than a horizontal beam, and the considerable height of the 
coelostat and object-glass above the ground may also prove 
advantageous. Should it prove successful, a similar instru- 
ment of larger aperture, and of about 150 feet focal length, 
may ultimately be constructed, on account of the importance 
of providing a very large image of the Sun for certain classes 
of spectroscopic and spectroheliographic work. It is prob- 
able enough that some other type of fixed telescope would be 
better than this, but the results of our experience up to the 
present time give reason for the belief that the present design 
will prove satisfactory. 

Since the principal difficulty to be overcome in the con- 
struction of a fixed telescope for solar work is the distortion 
of the mirrors by the Sun's heat, it is to be hoped that homo- 
geneous disks of fused quartz can ultimately be employed 
for mirrors, in place of glass. The coefficient of expansion 
of fused quartz is only about one-tenth that of glass, and 
hence it is but slightly subject to change of figure by heat. 
Many small quartz disks have been made in an electric fur- 
nace at the Solar Observatory, but the presence of numerous 
bubbles, which cannot be removed from the very viscous fluid 
by stirring, have proved an insuperable obstacle to the use 

iQr perhaps with fine wire netting, which should break the wind, and yet not 
heat sufficiently in sunlight to produce convection currents. 



234 



Stellae Evolution 




'^^^^t^^- 



FIG. 7 
Vertical Coelostat Telescope 



Some Possibilities of New Instruments 235 

of these disks for optical purposes. Day has met with better 
success in the geophysical laboratory of the Carnegie Insti- 
tution, where an electric furnace of special type permitted 
quartz to be fused under pressure. His results are suffi- 
ciently promising to lead to the hope that, if a large furnace, 
of suitable design, were constructed, disks of 15 to 20 inches 
in diameter might be made. In view of the expense of such 
a furnace, it has seemed best to defer further experiments 
in this direction until very thick glass mirrors can be 
thoroughly tested.^ 

Another important need of the future is a machine capable 
of ruling gratings of much larger dimensions than those of 
Rowland. The best Rowland gratings, which have rendered 
possible the great advances of the last quarter-century in 
spectroscopy, have a ruled surface about 5J inches long. 
The resolving power of a grating depends upon the total 
number of lines it contains, but there are many reasons 
why it is not desirable that the number should exceed 20,000 
per inch. If a good 15- or 20-inch grating could be ruled, 
with lines from 10 to 15 inches in length, a great advance 
in solar spectroscopy would be rendered possible. Such a 
grating, if one of the spectra were very brilliant, would be 
exactly what is required for a spectroheliograph capable 
of photographing the Sun through narrow dark lines. 
If used in a spectrograph of from 40 to 50 feet focal 
length, it would furnish a photographic map of the solar 
spectrum much superior to Rowland's, and be of the greatest 
service in the photography of sun-spot spectra, the study of 
the solar rotation, and many other investigations. For this 
purpose the spectra of one of the higher orders (from second 
to fourth) should be bright, and the precision of ruling 

1 Since the above was written the "tower telescope" has been constructed and 
tested. The thick mirrors are so little affected by sunlight that the focus will 
remain constant during the long exposures required with the 30-foot spectrohelio- 
graph (Plates CI and CII). 



236 Stellar Evolution 

should, of course, be so high as to permit the theoretical 
resolving power to be attained. In view of Michelson's 
recent work in ruling 8-inch and 10-inch gratings, the 
realization of his plan for the construction of a machine 
capable of making gratings of much larger size is more 
earnestly to be desired than that of any other project for 
the development of spectroscopy. 

Still another important need of the spectroscopist is 
homogeneous glass, in large masses, for prisms. At the 
present time it is almost impossible to obtain prisms of 
large size that will give good definition. The repeated 
failures of the best makers of optical glass indicate that the 
problem is not an easy one, though it can probably be 
solved. A careful study of this question, made with special 
reference to the possibility of improving the present methods 
of annealing, should yield valuable results. Large prisms 
are urgently required for use in stellar spectrographs of large 
aperture and high dispersion, such as the one which is to be 
mounted in a constant-temperature chamber in conjunction 
with the 60-inch reflector. However, if sufficiently large 
and perfect gratings can be obtained, which concentrate 
nearly all of the light in a single spectrum, they may be 
better for this purpose than prisms. 

In the further development of solar research, no instru- 
ment seems to offer more possibilities than the ^spectrohelio- 
graph. Recent experiments with a temporary spectrohelio- 
graph of 30 feet focal length, used in conjunction with the 
Snow telescope, have demonstrated the feasibility of photo- 
graphing sun-spots with the lines that are strengthened or 
weakened in their spectra. The resulting pictures show the 
distribution of the corresponding vapors in and around the 
spots, and should be capable of throwing much new light 
on solar phenomena when taken daily and systematically 
studied. It is expected that the 30-foot spectroheliograph 



Some Possibilities of New Instruments 237 

of the "tower telescope" will be employed in this way, but 
even the great dispersion of this instrument will be inade- 
quate for work with the finest lines. It is evident that if 
the photograph is to represent the distribution of the gas 
or vapor corresponding to the line employed, the line must 
be as wide as the second slit, in order that light from the 
adjoining continuous spectrum may not obliterate or confuse 
the image produced by it. When it is remembered that the 
solar spectrum contains more than 20,000 lines, and that 
any one of these may be capable of furnishing a photograph 
comparable in interest with the results already obtained with 
hydrogen and calcium lines, it will be appreciated that no 
effort should be spared to increase the dispersion and optical 
perfection of the spectroheliograph. The further applica- 
tions of this instrument to the study of the level of the 
flocculi; the absorption of the solar atmosphere; the growth 
of the flocculi and prominences, which can be shown, as if 
in accelerated progress, by the aid of a series of pictures 
taken in rapid succession and projected on a screen with a 
kinematograph ; the use of stereoscopic methods in spectro- 
heliographic work: these, and many other investigations, 
leave no doubt that this field of solar research is but barely 
opened, and still contains many untried possibilities. 

Passing over other considerations that tend to confirm 
one's optimistic belief in the future of solar research, we may 
now inquire as to the type of telescope that appears most 
promising for photographic and spectrographic studies of 
stars and nebulae. In much of this work it is not essential, 
as in the case of the Sun, that the image should be fixed in a 
laboratory. For this reason, an equatorially mounted reflect- 
ing telescope seems to meet the requirements admirably. 
Even when a fixed image is required, it is possible, as illus- 
trated in Fig. 2, Plate XCVII, to send the light from objects 
lying within a certain zone of the heavens into a constant- 



238 Stellar Evolution 

temperature laboratory, for analysis by the most powerful 
spectrographs. As already explained, such a telescope is 
also adapted for many other classes of work, either in the 
principal focus of the great mirror or with an enlarged 
image given by a convex mirror, after the manner of Casse- 
grain. 

As an object-glass increases in size, the absorption, due 
to its increased thickness, rapidly diminishes the percentage 
of light it transmits. The loss is especially serious for the 
blue and violet rays, since these are absorbed more completely 
than the red and yellow. In the case of a mirror, the light 
passes through no glass, but falls on a surface of pure silver, 
from which it is reflected to the focal plane. Thus every 
square inch added to the area of a telescope mirror means 
a proportional increase in the light-gathering power. It 
is evident that if the mechanical and optical difficulties 
can be overcome, reflecting telescopes much more power- 
ful than any now in existence can advantageously be con- 
structed. 

With this object in view, Mr. John D. Hooker has pre- 
sented to the Carnegie Institution a sum sufficient to pur- 
chase for the Solar Observatory a glass disk 100 inches in 
diameter and 13 inches thick, and to meet other expenses 
incident to the construction of a 100-inch mirror for a reflect- 
ing telescope of 50 feet focal length. The construction of a 
telescope so far surpassing all previous instruments in size 
must, of course, be partly in the nature of an experiment, i 
The immense block of glass will weigh 4^ tons, four and one- 
half times as much as the disk of the 60-inch mirror. The 
difficulty of providing a mounting capable of carrying it with 
the necessary precision is not slight. The glass is certain to 
be more or less distorted by temperature changes, which 
would ruin its performance if not obviated. The atmospheric 
conditions, even on Mount Wilson, may not be sufficiently 



Some Possibilities of New Instruments 239 

good to permit so great an aperture to be used to full advantage. 
Of these and other obstacles Mr. Hooker is fully informed, 
and he does not underestimate their importance. But he 
perceives and appreciates, with the understanding of one who 
has himself invented and developed mechanical appliances, 
that experiment is necessary to progress. He therefore does 
not hesitate to provide the means for undertaking an optical 
experiment on a large scale. Let us consider its probable 
outcome. 

In the first place, the question arises whether a sufficiently 
homogeneous glass disk of the required dimensions can be 
obtained. Our long experience with the Plate Glass Com- 
pany of St. Gobain (France) leads us to believe that no 
insuperable difficulty will be encountered. This old and reli- 
able company has cast for us scores of disks, from which 
Ritchey has made many plane and concave mirrors, from the 
smallest sizes up to 60 inches. In all of these cases the 
quality of the disks has left nothing to be desired. The 
60-inch, 8 inches thick, and weighing a ton, is fully equal to 
the smaller ones. We are therefore inclined to believe, since 
the St. Gobain Company expresses its deliberate opinion that 
a satisfactory disk, 100 inches in diameter and 13 inches 
thick, can be produced, that they will be able to carry out 
the order we have given them. 

As for the work of grinding and figuring, no one who 
has watched the progress of the 60-inch mirror would be 
likely to doubt Ritchey's ability to accomplish this difficult 
task. The method of parabolizing which he has perfected 
will apply as well to a 100-inch mirror as to the 60-inch, It 
eliminates the necessity of handwork, except for a few finish- 
ing touches, and has yielded an essentially perfect parabo- 
loidal figure in the case of the 60-inch mirror. I am con- 
fident that he will find no difficulty in bringing the 100-inch 
mirror to this highest order of perfection. 



240 Stellar Evolution 

The mounting should offer no great obstacles, especially 
as it will not be built until the mounting of the 60-inch 
has been thoroughly tested on Mount Wilson. In these 
days of large and perfect machinery, the mechanical diffi- 
culties are much less formidable than they would have 
appeared twenty years ago. On this score, therefore, we 
see no cause for fear. 

The prevention of change of figure due to changing tem- 
perature should not prove a very serious problem. During 
the fine nights of the best observing season on Mount Wilson 
the temperature remains almost perfectly constant after 9 P. M. 
It will therefore only be necessary to maintain the mirror 
(or possibly the entire telescope) at approximately this tem- 
perature throughout the day, by means of suitable refrigerat- 
ing machinery. In the long periods of cloudless weather the 
change of temperature from night to night is extremely 
small, so that little difficulty should be encountered on this 
score. If the slowly falling temperature during the early 
evening should prove to give trouble, the observational work 
may be deferred until after nine o'clock. The dome and 
building, like those for the 60-inch reflector, will be so con- 
structed that no air can enter during the day ; they will also 
be shielded from the heat of the Sun. The problem is, of 
course, altogether different from that encountered in the 
case of the Snow telescope, where the mirrors are required 
to give good images in spite of their exposure to direct 
sunlight. 

Assuming that these various difficulties can be success- 
fully overcome, it still remains a question whether the atmos- 
pheric conditions on Mount Wilson will be sufficiently good 
to permit the telescope to give satisfactory images. This 
cannot be definitely determined until after the 60-inch reflector 
has been used for some time. Even if it should prove, how- 
ever, that only a very few nights in the course of a year can 



Some Possibilities of New Insteuments 241 

be utilized to the fullest advantage, the construction of such 
a telescope would nevertheless be desirable. For under the 
average summer conditions, which are much finer than those 
in the eastern part of the United States, results of great value 
can undoubtedly be obtained in many classes of work, such 
as the photography of stellar spectra, the measurement of the 
heat radiation of the stars, etc. The immense amount of 
light which this mirror will collect should render it particu- 
larly suitable for spectroscopic work of all kinds. 

It need hardly be said that the 100-inch mirror, when 
suitably mounted, will play a most important part in the 
scheme of research of the Solar Observatory. The investiga- 
tion of stellar evolution frequently calls for adequate spectro- 
scopic study of stars beyond the reach of existing instru- 
ments. With the 4:0-inch Yerkes telescope, for example, it 
was impossible to obtain satisfactory evidence, positive or 
negative, as to the transition from solar stars to those of the 
fourth type. The large number of stars within the reach of 
a 100-inch reflector (which will give images about ten times 
as bright as the 40-inch) should greatly increase the chances 
of finding possible intermediate types, so important in their 
bearing upon the relationship of solar and red stars. This is 
only a single instance, but it forcibly suggests itself when con- 
sidering our programme of research. In other fields the large 
reflector should be equally valuable, especially for the pho- 
tography of the numerous small spiral nebulae, the details of 
which should be brought out to good advantage with a focal 
length of 50 feet; minute investigation of the larger nebulae, 
in the hope of detecting changes in their form ; the study, 
with very high dispersion, of the spectra of bright stars, etc. 
The remarkable calm of the summer nights on Mount Wilson 
should assist materially in all of this work, since vibration of 
the tube, caused by the wind, would undoubtedly be a 
serious drawback under less favorable conditions. 



242 Stellar Evolution 

It is impossible to predict the dimensions that reflectors 
will ultimately attain. Atmospheric disturbances, rather 
than mechanical or optical difficulties, seem most likely to 
stand in the way. But perhaps even these, by some process 
now unknown, may at last be swept aside. If so, the astron- 
omer will secure results far surpassing his present expecta- 
tions. 



CHAPTER XXV 

OPPORTUNITIES FOR AMATEUR OBSERVERS 

I SHALL never forget my delight, when as a boy, I first 
learned of the spectroscope. Its extraordinary achievements, 
and the endless possibilities, vaguely imagined, of its further 
applications in astronomical research, filled me with enthusi- 
asm, and kindled a strong desire for immediate work. The 
visual study of flames, with a simple one-prism spectroscope, 
aroused an ambition to photograph spectra. This was soon 
accomplished, by substituting an ordinary camera for the 
observing telescope. But the scale of the photographs was 
too small, so I built a longer camera of wood. Later, when 
Rowland was making his earliest gratings, one of the small- 
est size was secured, and substituted for the prism. The 
marvelous increase in resolving power, and the greatly aug- 
mented beauty of the solar spectrum, led to observations of 
the solar prominences, and subsequently to more serious 
research. But none of the pleasures of later years, during 
which I have enjoyed the privilege of using larger and more 
powerful instruments, has surpassed the delight of the initial 
work, much of which was done with simple and inexpensive 
apparatus of my own construction. 

These remarks are called forth by certain criticisms I have 
heard of great modern observatories. Some amateurs, I 
am told, believe that their efforts are rendered futile by 
the more powerful equipment and better atmospheric advan- 
tages of other investigators. If this feeling were well- 
grounded, it might fairly be asked whether the great observa- 
tories are worth their cost. For the history of astronomy 
teaches that much of the pioneer work has been done by 

243 



244 Stellak Evolution 

amateurs, usually with modest means and in unfavorable cli- 
mates. To discourage this class of workers, unfettered as 
they are by the traditions of institutions, and driven by their 
own initiative into unexplored fields, would be a serious error, 
hardly to be atoned for by any services the larger observa- 
tories can render. 

We may therefore inquire whether useful work, of such 
a nature as to contribute in important degree to the progress 
of science, can still be done with simple and inexpensive 
instruments. This question may at once be answered in the 
affirmative. The results of amateur observations may not 
only be useful — they may equal, or even surpass, the best 
products of the largest institutions. Great care must be exer- 
cised in choosing the subject of research, in constructing the 
instruments, in making the observations by the best methods 
and at the most favorable hours, and in the reduction and 
discussion of the results. If such precautions are observed, 
discouragement will soon give way to confidence and success. 

Take, for example, the direct photography of the Sun. A 
2-inch objective, of 40-feet focal length, will give beautiful 
solar photographs, over 4 inches in diameter, perfectly 
adapted for the study of the solar rotation, the proper motions 
of the spots, and other important purposes. Details sepa- 
rated by less than two seconds of arc will not be resolved on 
these photographs, but in many classes of work little gain 
would result from increased resolving power. Such an ob- 
jective should be mounted so as to send the beam horizontally 
(better vertically) across shaded ground, or within a building, 
to the photographic plate. If no coelostat is available, a small 
mirror, with optically plane reflecting surface, will serve the 
needs of direct photography. It is only necessary to mount it 
on a wooden support, so that it can be held at the angle required 
to reflect sunlight through the objective. The exposures — 
made by the rapid motion of a wooden shutter, pierced by a 



Opportunities for Amateur Observers 245 

narrow slit with brass edges, mounted jnst in front of the 
plate — are very short, and the slight drift of the solar image 
during this time can be overcome, when desired, by a very 
simple driving mechanism. Between exposures the small 
mirror should be shielded from the Sun. The apparatus used 
by the American parties to photograph the last transit of 
Venus across the Sun was of this type, except that a 4-inch 
objective and larger mirror were used. 

It w411 probably be found that the best solar definition 
occurs in the early morning, before the ground is greatly 
heated. A careful study should be made of the local con- 
ditions before selecting the hours of work. 

Solar photographs, made in this way at intervals of from 
one to several hours, may be combined in the stereoscope 
with striking results. More important, however, would be 
a long series of photographs, made at short intervals, and 
examined with a kinetoscope. These should show the Sun 
rotating under one's eyes, the spots near the equator moving 
more rapidly than those in higher latitudes. The effect of 
proper motion, in causing some spots to overtake others in 
the same latitude, should also be very finely brought out. 
Even more interesting, however, would be the changing 
forms of spots, and the manner of their growth and decay, 
which have never yet been observed by this method. 

The same horizontal telescope, with some modifications, 
would give an admirable image for spectroscopic work. The 
objective should, if possible, be of from 4 to 6 inches aperture, 
and from 40 to 60 feet focal length. The mirror should also 
be increased in the same ratio, and mounted as a coelostat, 
with its plane parallel to the Earth's axis. If the mirror is 
very thick — 3 inches or more — its form will be changed but 
little by sunlight. A second mirror will be needed to send 
the beam to the spectrograph, as in the Snow telescope 
(Plate LVIII). If this arrangement appears formidable, it 



246 Stellae Evolution 

should be remembered that almost all the parts can be made 
of hard wood, thoroughly soaked in melted parafhne, to pre- 
vent warping. The bearings are practically the only parts 
that need be of metal. A cheap clock movement, with heavy 
spring, will serve for a driving-clock, or a small electric 
motor may be used. With moderate ingenuity, any amateur 
accustomed to the use of tools can build such an instrument 
for a very small sum. 

The spectrograph is even more simple. It should be of 
the Littrow form (p. 153), and the aperture of the single 
plano-convex lens that serves for both collimator and camera 
should be from 1 to 1-| inches. Its focal length will be 
determined by the diameter and focal length of the objective 
used to form the solar image on the slit. If these are 4 
inches and 60 feet, respectively, the ratio will be 1:180. 
Hence the focal length of the spectrograph lens should be 180 
times its aperture, or from 15 feet to 22 feet 6 inches. The 
grating should be a 2-inch Rowland, or, if this is too expen- 
sive, a good replica by Ives, Wallace, or Thorpe. The repli- 
cas have the disadvantage of being made on transparent films, 
for use with transmitted light ; but they can perhaps be con- 
verted into reflecting gratings by silvering. 

The collimator-camera lens should be mounted on a ver- 
tical wooden bracket, arranged to slide 3 or 4 inches for 
focusing. The grating may also have a wooden support, 
consisting of a bracket, which can be tipped forward or 
back, mounted on a circular wooden table, permitting rota- 
tion about a vertical axis in the plane of the grating. Such 
rotation is necessary in order to bring different spectra upon 
the photographic plate, or to pass from one region to another 
in the same spectrum. The height of the spectrum on the 
plate can be adjusted by tipping the grating forward or back. 
It is also necessary to make the lines of the grating parallel 
to the slit; this can easily be done by hangiDg the bracket 



Opportunities for Amateur Observers 247 

from above, and defining its position by two side screws, 
passing through wooden blocks attached to the circular table. 
Plate cm shows a wooden lens and grating support in reg- 
ular use as part of a Littrow spectrograph of 18 feet focal 
length in the laboratory of the Solar Observatory. 

The extreme simplicity of the slit end of the same instru- 
ment is illustrated by Plate CIV. A short slit, with one jaw 
movable by a screw, is supported by a tube fitting tightly in 
a hole bored through a wooden bracket. Below is the plate- 
holder, held in a frame that slides up and down, permitting 
many narrow spectra to be photographed on the same plate. 
In another similar instrument the slit and plate-holder sup- 
port stands on a pier, and fits into a partition, so as to exclude 
all light from the room except that which enters through the 
slit.' In this case no tube is necessary between the plate and 
lens. The latter is mounted, with the grating, on a pier at a 
distance from the slit equal to the focal length of the lens. 

In spite of the simplicity and cheapness of such a spectro- 
graph, no better instrument could be asked. Its one draw- 
back — the reflections of the slit from the surfaces of the lens 
— is easily removed by placing a bar across the lens (as shown 
in Plate CIV) . Wooden spectrographs are in constant use 
at the Solar Observatory, and give results which are very 
satisfactory. 

Any of the solar spectroscopic work described in this book 
can be done with such an instrument. The resolving power, 
even with only an inch aperture, will be sufficient for the sepa- 
ration of very close solar lines. The spectra of sun-spots, the 
solar rotation, the remarkable differences between the spectra 
of the center and limb of the Sun, and many other phenom- 
ena can be studied by its aid with the greatest precision and 
success. The exposures, it is true, must be longer than with 

1 This room is part of a long hall, for testing optical mirrors, in the Pasadena 
shop of the Solar Observatory. By opening large light-tight doors, the hall can be 
used for the transmission of light in the knife-edge tests. 



248 Stellar Evolution 

a spectrograph of larger aperture, but this is not a serious 
obstacle. Indeed, it may be said that at the present time 
only two or three observatories in the world are using equip- 
ment as powerful as this for the classes of solar work just 
enumerated. 

I might go on to describe a wooden spectroheliograph, 
fitted up with spare lenses and prisms, which gave excellent 
results with the Snow telescope before the 5-foot spectro- 
heliograph was completed. Indeed, the photographs were 
quite equal to those taken with the latter instrument, except 
that they did not include the entire solar image, which is 
unnecessary for many kinds of work. The small coelostat 
telescope described above would give as good results as the 
Snow telescope with such a spectroheliograph, except that the 
exposures would be longer. The entire apparatus is easily 
within the reach of any intelligent amateur of limited means. 

Those who desire to undertake solar work would do well 
to procure the Transacflons of the International Union for 
Co-operation in Solar Research.^ The aim of the Union is 
to encourage co-operation among observers, in the various 
fields where this is desirable. For example, it is impossible, 
in visual observations of sun-spot spectra, for one person to 
make a thorough study of more than a limited region. By 
mutual agreement, the spectrum is therefore divided up 
among many observers, who record their results on a common 
plan. Spectroheliographs, distributed from India across 
Europe to California, are also operated in harmony, and co- 
operation is practiced in other fields as well. Apart from 
such routine, however, every observer is encouraged to act 
on his own initiative, for the Solar Union recognizes that 
the greatest advances will come from individual effort, which 
no amount of co-operation can replace. 

1 Vol. I was published by the University Press, of Manchester, England, in 1906. 
Vol. II will soon appear. 



INDEX 



Abbot: tests of Mount Wilson atmos- 
phere, 129; solar radiation, 214; pyrhe- 
liomc ter, 215. 

Absorption: spectrum, 52; in solar 
atmosphere. 53, 68 ; in hydroerpnflocculi, 
96; in stellar atmospheres, 173. 

Adams: metallic and spot spectra, 159; 
titanium oxide in spots, 162 ; spectrum 
of Areturus. 168 ; spectrum of a Orionis, 
170; Trapezium stars, 189; "Orion" type 
stars, 190. 

Altitudes : advantages of high, 111-20. 

Amateurs: opportunities for, 27, 243-19. 

Andromeda nebula, 41, 44, 

Anomalous dispersion and solar phe- 
nomena. 148. 

Antares, 195. 

Arcturus: spectrum, 168; heat radiation, 
172, 173. 

Astrophysics : relation to astronomy and 
physics, 6. 

Atmosphere : absorption in Earth's, 63, 
114, 128; unsteadiness, 111, 112, 127. 

Barnard: photography of Milky Way, 
30-33, 128; micrometric observations, 
103 ; comparative photographs atMount 
Wilson and Lake Geneva. 128, 129 ; tests 
of Mount Wilson definition, 129. 

Barnard and Ritchey : photography 
of corona, 76. 

Betelgeuze: spectrum, 170. 

Binaries : spectroscopic, 105. 

Bolometer, 215. 

Boys : stellar heat, 171. 

Bruce spectrograph, 104, 167. 

Bruce telescope, 29-33. 

BuRNHAM : discoveries with small tele- 
scope, 27; observations with Verkes 
refractor, 103. 

Calcium : lines, H and K, 84, 91 ; flocculi, 
85-93, 143, 147 ; vapor, radial motion of, 
92. 

Calcium hydride: m sun-spots, 163. 

Calvert: corona, 73. 

Camera lens : stellar photography with, 
28-32. 

Campbell : stellar motions, 105. 
Canes Venatici : spiral nebula in, .38, 39. 
Carbon: in chromosphere, 80; in red 

stars, 195. 
Carnegie, Andrew: establishment of 

Carnegie Institution, 109. 



Carnegie Institution : purpose of, 109. 

Chamberlin : criticisms of nebular 
hypothesis, 182-86; planetesimal hy- 
pothesis, 208-10. 

Chromosphere, 15, 84; spectrum, 78; 
"flash" spectrum, 80. 

Coelostat, 75, 109, 245; advantages, 131; 
Snow telescope, 133, 134; "tower" tele- 
scope, 231. 

Co-operation in research, 98, 218, 249. 

CoRNU: telluric lines, 63, 64. 

Corona, 16, 73-75; spectrum of, 74. 

Crossley reflector, 42, 45, 

Cygmis : nebula in, 44. 

Darwin, Charles : Origin of Species, 1 ; 
correlation in research, 97. 

Darwin, Sir George: tidal friction, 183; 
meteoroidal swarm, 183, 204. 

Deslandres : level of calcium flocculi, 
90; spectra of flocculi, 96; spectre -helio- 
graph, 96; Foucault siderostat, 131. 

Draper, 41, 54, 

Echelon, 65. 

Eclipse: solar, 73; apparatus. 75. 

Ellerman : work with Kenwood spectro- 
heliograph, 86; work with Rumford 
spectroheliograph, 89 ; work with 5-foot 
spectroheliograph, 138 ; photography of 
spot spectra, 152, 163, 

EvERSHED : spectroheliograph, 96. 

Evolution : early views, 1 ; general 
problem, 3. 

Faculae, 15, 71, 72, 85, 86, 90, 146. 

Flagstaff. 123. 

"Flash" spectrum, 80. 

Flocculi: calcium, 85; daily motion, 87, 
142, 146; minute, 89 ; levels, 90; eruptive, 
92; hydrogen, 94; iron, 96; h' liographic 
positions. 144 ; proper motions, 146 ; level 
of calcium and hydrogen, 147 ; levels, 
150; areas, 150, 217. 

Foucault: siderostat, 131. 

Fowler: magnesium hydride in spots, 
163; titanium oxide in red stars, 195. 

Fox : measures of Kenwood plates, 144. 

Fraunhofer : dark lines in solar spec- 
trum, 47 ; objective prism, 189; stellar 
spectra, 189. 

Frost: stellar spectroscopy, 104, 105; 
heat radiation of sun-spots, 149 ; Trape- 
zium stars, 189; '"Orion''' type stars, 190. 

Furnace: electric, 160. 



249 



250 



Stellae Evolution 



Gale: metallic spectra. 159. 

Galileo: early discoveries, 9. 

Globe measuring machine, 144. 

Gratings: Rowland, F)6-.59, 23.i, 236; 
Michelson, 65, 66, 2.36; Jewell, 63. 

Greenwich Observatory: spot posi- 
tions, 144. 

Harvard Observatory : refractor, 41 ; 
objective prism, 189; stellar specira, 
201. 

Heliomicrometer, 144. 

Helium: in Sun, 78, 79; terrestrial, 78; 
in ''Orion'' stars, 79; in nebulae 190. 

Herschel, Sir John : clusters and 
nebulae, 46. 

Herschel, Sir William: clusters and 
nebulae, 46 ; condensation of nebulae, 
187. 

HiGGS: map of solar spectrum, 62. 

Hooker, J. D. : gift of lOO-inch mirror, 
238. 

Hooker expedition, 30, 128. 

Hooker telescope, 238-42. 

HuGGiNS: stellar spectra, 53; promi- 
nences, 54, 76; spectrum of nebulae, 54; 
helium, 79; stellar motions, 105; stellar 
heat, 171; stellar evolution, 199; tem- 
perature of nebulae, 207. 

Hydrogen: spectrum, 78; in stars, 79, 
170, 191, 193, 195, 199, 200, 208 ; in promi- 
nences. 83; in nebulae, 190; in meteor- 
ites, 20.. 

Hydrogen flocculi, 93-95; level, com- 
pared with calcium, 147. 

Interferometer, 65. 

Janssen: prominences, 54, 76; solar 

photography, 70. 
Jewell: telluric lines, 63; gratings, 66. 
Julius: anomalous dispersion th( ory, 

148. 

Kapteyn: structure of universe, 202. 

Keeler: spiral nebulae, 3, 45; pho- 
tography with Crossley reflector, 42; 
Saturn's rings, 182; chief nebular line, 
205. 

Kenavood Observatory, 83; spectro- 
heliograpli, 84; spot spectra, 1.52. 

Kirchhoff: explanation of solar spec- 
trum, 51-53. 

Kirkwood: nebular hypothesis, 185. 

KODAIKANAL OBSERVATORY, 119. 

Laboratory : Yerkes Observatory, 107 ; 
Solar Observatory, 156. 

Lane's law, 191. 

Langley: sun-spots, 69; photospheric 
grannies, 69-71 ; color of Sun, 193 ; solar 
radiation, 214; bolometer, 215. 



Laplace: nebular iiypotliesis, 2, 175-86. 

Lick Observatory, 42, 119, 120, 205. 

Lick telescope, 26, 41. 

Littrow spectrograph; laboratory, 
156; of Snow telescope, 1.34, 153; of 
"tower" telescope, 232; wooden, 246-48. 

Lockyer: prominences. 54, 76; helium, 
78; sun spot spectra, 151; dissociation 
in sun spots, 151; temperature of sun- 
spots, 152; temperature of stars. 173; 
enhanced linei?, 194; stellar classific i- 
tion, 194, 207; meteoritic hypothesis, 
204-8. 

Magnesium hydride: in sun-spots, 163. 

Magnifying power, 22. 

Mars: period of inner satellite, 183. 

Maunder: band lines in spots, 152. 

Maxwell: Saturn's rings, 182. 

Meteorites : spectra, 205. 

Michelson: interferometer, 65; stand- 
ard wave-lengths, 65, echelon, 65; 
gratings, 65, 66, 236. 

Milky Way: photographs of, .30-33. 

Mills spectrograph, 167. 

Mirror : 60-inch, figuring. 219-26 : method 
of ti sting, 222, 224-26; 100-inc i, 238-41. 

Mirrors : distortion, 1.37, 138, 231-35, 240. 

Momentum : moment of, 185. 

Moon : photography of, .33. 

Mont Blanc, 119. 

MouLTON : criticisms of nebular hypoth- 
esis, 182-86. 

MouLTON AND Chamberltn : planetesi- 
mal hypothesis. 208-10. 

Mount Etna: expedition to, 116-19. 

Mount Hamilton, 119. 120. 

Mount Wilson, 123-30. 

Mount Wilson Solar Observatory: 
origin, 110; plan of research, 121; site, 
123-30; Snow telescope, 131-3S; work 
with spectroheliograph, 1.39-.50; sun- 
spot spectra, 153-64 ; laboratory, 155-58, 
160; stellar spectroscopy. 16 (-71; 60-inch 
reflector, 219-29; "tower" telescop , 
232-35; lOO-inch reflector, 238-42. 

Mountains : as observatory sites, 113-30. 

Nebula : spiral in Canes Venatici, 38, 39; 
in Andromeda, 41, 44,45; in Cygnus, 44; 
in Orion, 44, 188-90; in Draco, 54; La- 
place's, 177. 

Nebulae: spiral, 7, 38, 39, 41, 44, 45, 188, 
203; relationship to star-, 3, 32. .55, 177, 
187-90, 198-201, 206, 207, 209, 210; and 
clusters, 18, 46, 47, .54; in Milky Way, 31, 
32; in Pleiades, 44, 129, 198, 202; spec- 
trum of, 54, 190, 208; condensation, 
17>-81, 183, 187, 200,201,212; planetary, 
187; temperature of, 207. 

Nebular hypothesis, 2, 175-88. 

Nebulum, 204. 

Neptune: orbits of satellites, 183. 



Index 



251 



Newton : analysis of tiunlierht, 47 ; advan- 

taires of high altitudes. 111. 
Nichols : stellar heat, 172. 

Olmsted: calcium hydride in spots, 163, 

Oru/in of Species, 1. 

Orion nebula, 44, 188-90; Trapezium 

stars, 188, 190. 
""Orion''' TYPE STARS, 79, 190. 

Photography: advantatres, 28; star 
trails, 29; with camera, 29-3.3; clusters, 
33, 44; Moon, 83, 34; with Yi-rkes tel- 
escope, ,3.3-.36; with reflectors, 4l)-4o; of 
nebulae, 41-45, 203; solar spectrum, tO- 
64, 247, 24X; Sun, 70-72, 244, 245; eclipse, 
74-76; with spectmheliograph, 81-96, 
139-42. 149, 150, 236, 237, 248; stellar 
spectra, 104, 1U5, 165-71. 189-97, 203 ; coro- 
na, without eclipse, 115-18; Milky \\ ay, 
128, 129; with Snow telescope. 137, 1.38; 
sun-spot spectra, 1.52-54; metallic spec- 
tra, 158-62. 

Photosphere: structure, 69-72. 

Physics: fundamental importance of, 5. 

Pic du Midi, 119. 

Pickering, E. C: objective prism, 189; 
stellar spectra, 201. 

Pike's Peak: expedition to. 115, 116. 

Planetesimal hypothesis, 208-10. 

Pleiades, 18, 44, 129, 177, 198, 202. 

Potsdam Astrophysical Observatory, 
100, 167. 

Prism: formation of spectrum, 48; ob- 
jective, Fraunliofer, 189; objective, 
Pickering. 1^9; glass. 236. 

Prominences, 15; nature of, 76; without 
eclipse, 77; spectrum, 78; seen with 
open slit, 81; quie-ceut and ■ ruptive. 
81, 93; photography of, 82; H and K 
in, 84. 

Pyrheliometer, 215. 

Quartz: fused, for telescope mirrors, 
23.3-35. 



Eadiometer, 172. 

Eadium, 212, 213. 

Kamsay : discovery of helium, 78. 

Reversing layer, 143. 

Ritchey: photograp' s of Moon, 33, 34; 
photography with Yerkes telescope, 33, 
36; 24-inch reflector, 43; phot'grapi.y 
with reflector. 44; telescope construc- 
tion, 43, ^19-30; 100-inch reflector, 238-40. 

Ritchey and Barnard: pljotography 
of corona, 76. 

Roberts: Andromeda nebula, 41. 

Rosse: 6-foot reflector, .38, .39. 

Rotation: solar, by spots, 143; by facu- 
lae, 143; by flocculi, 143, 146. 

Rowland: gratings, 56-59; composition 
of Sun, 62; map of solar spectrum, 62; 
solar spectrum wave-lengths, 62. 



Rumford spectroheliograph, 88. 
Runge: helium, 79. 

Rt'THERFURD: stellar spectra, 2, 53; 
gratings, 57. 

Saturn: ring system, 179; constitution 
of rings, 182; revolution of rings, 184. 

Schuster: stellar evolution, 199. 

Secchi: st liar spectra, 53; prominences, 
76 ; classification of stellar spectra, 170. 

Sirius: spectrum, 191. 

Smith, Piazzi: Tenerili'e expedition, 119. 

Smithsonian Observatory, 214. 

Snow telescope, 132-38. 

Solar Union, 218, 248. 

Spectra: stellar. 2, 18, 104. 105, 164-71, 
18^-91, 19.3-203, 207, 208; nebulae, 18, 54, 
203-7; solar. 47, 51-53, 60-64, 215; con- 
tinuous, 49, .51; bright-line. 49-53. 60. bl, 
107, 141, 157-63, 105; dark-line, 51-54. 93, 
94; prominences, 54, 76-81; eratini.'-, 5>- 
60: chromosph' re, 78-81: "flash," 80; 
flocculi, 85, 86, 90. 91, 9.3-96; faculae, 90; 
sun-spots, 108, 151-64 ; a ro, 1.59-62; elec- 
tric furnace, 160, 161 ; Satur))\s ring, 182 : 
Sun. c nter and limb, 192; "enhanced" 
lines, 194. 

Spectrograph : Bruce, 104, 167 ; Littrow, 
1.34. 153; laboratory, 1.56; Mills, 1^7; 
Potsdam, 167; grating, for stars, 167, 
228; of "tower" telescope, 232; wonden, 
246-48. 

Spectroheliograph: principle of. 82; 
Kenwood, 84; Rumford, 88; use of dark 
lines, 93; .5-foot. 139; operation. 141; 
.30-foot, 233 ; future development, 23H. 

Spectroscope: Kirchhoft's, 51; plane 
grating, 58, 77; concave grating, .59, 60; 
objective prism, 80, 189. 

Spectroscopic binaries, 105. 

Spencer : nebulae, 47. 

Spurious disk, 23. 

Stars: clusters, 18, 46; colors, 1^,170, 173, 
191, 193, 195, 199; size of image, 23; 
clouds, 31; spectra, 53, 104, 105, 16''-71, 
173, 174, 189-203, 207, 208, 241 ; tw nkling, 
111; heat radiation, 171; red, 173, 195- 
97.208; temperature, 173; helium, 190; 
"'Orion'' type, 190; white, 191 ; dark, 197. 

Stereocomparator, 147. 

Sun: visual appearance, 15,68; composi- 
tion, 16; activity, 16, 217; as a star, 17; 
line absorption, .53; absorption in at- 
mosphere, 68; direct photography, 70, 
245; inclination of axis, 143; rotation, 
143, 146; contraction, 191; spectra of 
center and limb, 192; radiation, 212-16. 

Sun-spots, 15, 69; periodicity, 16; level, 
71, 148; heliographic positions, 14.3- 6; 
dissociation in, 1.51; darkness, 151, 163; 
spectrum, 151-64 ; temperature, 163. 

Telescope: magnifying power, 22; 
brightness of image, 22: resolving 
power, 23; large and small, 24-27 ; fixed, 
131,2.32; "tower," 232. 



252 



Stellar Evolution 



Telescope: reflecting, Rosse. 38; devol- 
opmentof, 38-45 ; advantages, 42 ; ( 'ross- 
ley, 42, 45; 24-inch, 43; Snow, 132-38; 
60-inch, 219-30; lOU-inch, 238-42. 

Telescope: refracting, 21, 230; Yerkes, 
25, 26, 33-36, 43, 88, 101-4; Lick, 26, 41; 
Burnham's 27; camera, 28-'3; Bruce, 
29, 30; Kenwood, 33, 84; development 
of, 41. 

Telluric lines, 63, 64, 

Tenekiffe expedition, 119. 

Titanium oxide: in sun-spots, 162. 

"Tower" telescope, 232. 

Trapezium stars, 188, 190. 

Turner: coelostat, 132. 

Twinkling of stars. 111. 

Uranus: orbits of satellites, 183. 



Vega: heat radiation, 172, 173. 
Vogel: stellar motions, 105. 

Wadsworth : reflector mounting, 43. 
Wilson: sun-spots as cavities, 71. 

Yerkes Observatory : policy, 98 ; origin, 
99; plan of building, 100; instrument 
and optical shops, 106; spectroscopic 
laboratory, 107; site, 113; coelostat 
room, 172. 

Yerkes refractor : photography, .33-36, 
88; mounting, 34, 101; compared with 
reflector, 44; objective, 101; operation 
of, 102. 

Young: discovery of "flash'' spectrum, 
80; prominencrs, 81 ; H and K in prom- 
inences, 84; calcium flocculi, 85; pho- 
tography of spot spectra, 152. 



PLATE II 




Direct Photograph. Showing the Sun as it Appears to the Eye 



PLATE III 




The Solak Cheomospheee and Prominences 



PLATE IV 



^ 


f 


^ I i ^^1 


F 

i . 


i ii^ 






(■■III ... 



Fig. 1 
Characteristic Spectra of (a) White, (b) Yellow, and (c) Red Stars 

(Huggins) 




Fig. 2 

The Solar Corona 

Photographed by Yerkes Observatory Eclipse Expedition, May 28, 1900 (Barnard and Eitehey) 



PLATE VI 




Star Trails Photographed with 2i^-inch Portrait Lens 
(Ritchey) 



PLATE VII 




The Bruce Telescope of the Yeekes Observatory 



PLATE VIII 




Star Cluster Messier 11 and the Surrounding Milky Way 
Small-scale photograph taken with lantern lens (Barnard) 



PLATE IX 




Star Clustee Messier 11 and the Surrounding Milky Way 
Larger-scale photograph taken with 10-inch Bruce telescope (Barnard) 



PLATE X 




The Milky Way near p Ophiuchi 
Photographed with lU-inch Bruce telescope (Barnard) 



PLATE XI 




Star Cluster Messier 11 
Large-scale photograph taken with iO-inch Yerkes telescope (Ritchey) 



PLATE XII 




The Moon 
Photographed with the 12-inch Kenwood refractor (Ritchey) 



PLATE XIII 




Lunar Crater Theophilus and Surrounding Region 
Photographed with the 40-inch Yerkes refractor (Ritchey) 



PLATE XIV 




The 40-inch Refkactor of the Yekkes Observatoky 



PLATE XVI 




90-FOOT Dome of the Yeekes Obsekvatoky 



PLx\TE XVII 




Eye-End of Yekkes Telescope 
Showing double-slide plate-holder 



PLATE XVIII 




The 24-inch Reflector of the Yerkes Observatory 



PLATE XIX 




Stab Clustek Messier 13 
Photographed with the 24-inch reflector of the Yerkes Observatory (Ritchey) 



PLATE XX 




Star Cluster Messier 13 
Photographed with the 40-inch Yerkes refractor (Ritchey) 



PLATE XXI 




The Great Nebula in Orion 
Photographed with the 24-inch reflector (Ritchey) 




s ^ 

H " 

b H 




1^ S 



PLATE XXIII 




SiK WiLT.IAM HUGGINS 




w 



M OIST A 



DRY AIR 









i-f;:-. 




.K 










0- 


« 








-< 


> 


^^{!C = 


rF 


X 


o { '- 


— 'O 


H 






— z 

- 2 

.1 


K 
B 




,t fl' ■- 




r" 


^ 


^ 






Z 


Ph 


^W— 




h^ 




r "*■-- 


"* 


2 




-^ / '--- 














> 










H 
< 




^ { "^ 










5 




tK:> 




















X 

o 






















<u /:"■->, 
















-r) |~ - - -- 


" ' 























o t :■ ^ 




u 










"~° . 






m r- ^ 








ta ; _/ 










miiiiiiiiiiiHBi 




























MOIST AIR DRY AIR 



PLATE XXVI 




Langley's Drawing of the Typical Sun-Spot of December, 1878 




H ^ 



X 




PLATE XXXII 



(«) 



ib) 



(c) 




Beight H and K Lines on the Disk (a, 6, and c), in the 
Chromospheke (6), and in a Prominence (a) 



PLATE XXXIV 




Spectkoheliogkaph Attached to 12-inch Kenwood Refbactok 



PLATE XXXV 




Eruptive Prominence Photographed with the Kenwood Spectroheliograph 
March 2o. 1895, lOli 40m. Height of prominence, 162,000 miles 



PLATE XXXVI 




Eruptive Prominence Shown in Plate ;XXXV Photographed 18 Minutes Later 
Height of prominence, 281, OCO miles 



PLATE XXXVII 




RUMFORD SpECTROHELIOGKAPH ATTACHED TO iO-INCH YeRKES ReFRACTOR 



PLATE XXXVIII 






The Sun, Showing the Calcium Flocculi 
August 12, 1903, 81i 52m 




O 03 

td J" 






8 s 



PLATE XL 



















Fig. 1— 3h 40m. Second slit set on Hi 




Fig. 2.— 3I1 Sim. Second slit set on H2 Same region of the Sun 
as that shown in Fig. 1 

Minute Steuctuhe of the Calcium Flocculi 
September 22, 1903. (Scale : Sun's Diameter = 0.890 Meter) 



PLATE XLI 



•Mi «« f* I ^ ^'Wll 




Fig. 1 
Pkism Tkain of the Rumfokd Spectkoheliogeaph 




Fig. 2 
H AND K Lines of Calcium in the Electric Arc 



PLATE XLIII 

N 




Fig. 1.— 3h 57m. Calcium flocculi (K2) 



W 




Fig. 2.— llh Om, Hydrogen flocculi ( Ey) (briglit eruptive flocculi west of spot) 
Hydrogen and Calcium Flocculi, July 7, 190;^ 



PLATE XLVI 




M 



The Bruce Spectrograph of the Yerkes Observatory 
Mounted on its carriage, with constant temperature case removed 



!^ 8 



X 



W 




^ H 



Wf^ 



^ < 



rS H k: 



c-i O 



fe X 



;f>W^ 





< o 

^ I 



H 



P-i 




PLATE LVI 




Fig. 1.— At the Yerkes Observatory. Exposure 40'" 




Fig. 2.— At Mount Wilson. Exposure 41'>i 

Star Cluster Messier 35 

Photographed with the Bruce telescope (Barnard) 



>< 




PLATE LXIII 




DiEECT PhOTOGKAPH OF THE SuN 

August 25, 1908, 6li 09i" A. M. 



PLATE LXIV 




The Sun, Photogkaphed with the 5-foot Spectroheliograph 

August 25, 1906, 6^ 22m a. m. 

Camera slit set on Hi line of calcium 



PLATE LXV 




The Sun, Photogkaphed with the 5-foot Spectroheliograph 

August 2o. 1906, 6li 18m a. m. 

Camera slit set on Hg line of calcium 



PLATE LXVI 




The Sun, Photographed with the 5-foot Spectroheliogkaph 

August 25, 1906, 6li 86i» A. M. 

Camera slit set on Hs line of hydrogen 



PLATE LXVII 




The Sun. Photogkaphed with the 5-foot Spectkoheliogeaph 

August 25, 1906, bh 28"! A. M. 

Camera slit set on the iron line A 4046 



PLATE LXVIII 




i^S 



i' 



> V 



Fig. 1.— 7h 46'". Hydrogen Flocculi 



Fig. 2.— 7ii oJlm. Iron Flocculi 
(A 4046) 



Hydrogen and Ikon Flocculi Photographed with the 5-foot Spectroheliograph, 

November 13, 1907 



PLATE LXX 




The Heliomickometek 




« XT 



o P 



*F= 








I « " 

H C 

^ 'i 

O ^ 

^; 




Ph 




n 

o 






t 








_.,__: 


o — 






o — ■ 

00 




H 


~_: 


MHlMv 


lHni 


o — 

rv .. . 




■■ 






BB 


__.. 






o — 
o 




^1 


o — 




M 


— 




fcr 


— 






310 4 

!lii'h!l Mil i; 


















o 







r 




M 




^ 








E^ 




?H 




7J 




fH 




o 




CL 




C/3 




^ 












7J 




pa 




X 




H 




P^ 




O 


^ 






Ph 


--s 


^ 




^ 








o 








K 


,u 


Ph 




-i! 


a 


M 








o 




^ 


M 


M 


'~- 


P^ 


--- 


>^ 


--' 


M 


^^ 



^:! 




(^ 


o 


Q^ 


_a; 




lo 


Pm 


o 





^ 


-5! 


"^ 




^ 


U 


t3 


w 





fM 


M 


gq 


A 




c« 




i-i 




!JC 









.(^ 




O 




ja 




Ph 



tq — 






O cs 




H ^ 



•2 ^ u 

? H c £ 






s .^ 







?^ 






^ „-^^._ 


n: 


. ..__ 




■n^-—. 




::^ 


i^- 


4 


-=r= _- 


-^ - J 








1 




— '"5 


— 


3 






^ 






1 






^ 






■i^ 




H|H 


^^^^1 






^^H 


^^^^H 









PLATE LXXXIII 



5300 



r)400 



5500 5600 5700 




Carbon 
Arc 



132 Schj. 
(IV)' 



a Orion'n 
(III) 



Fig. 1 

Region of Yellow Caebon Fluting in Electric Arc, Fourth 

Type Star (182 Schjellerup), and Third 

Type Star (a Orionis) 




Fig. 2 

Spectra op Four Fourth Type Stars 

Photograp'iei with the 40-inch Yerkes refractor, showing how the dark carbon 

band becomes stronger as the star cools 








o 

O CO 
o < 



'"wl <! m 



5 O 

C5 n 



H < 



PLATE LXXXVI 




The Pleiades 
Photographed with the 2-t-inch reflector of the Yerkes Observatory (Ritchey) 



PLATE LXXXVII 




Nebula in Cygnns, N. G. C. 6992 
Photograpbed with the 24:-inch reflector (Ritchey) 



PLATE LXXXVIII 




Sfikal Nebula Messier 51 Caniim Venaticoi^uni 
Photographed with the 24-inch reflector (Ritchey) 



PLATE LXXXIX 




Spiral Nebula Messier^ 101 
Photographed with the 24:-inch reflector (Ritchey) 



PLATE XC 




Spiral Nebula Messier 33 Trianguli 
Photograplied with the 24-inch reflector (Ritchey) 



^^^^^^^^^^^m 




PLATE XCIV 




)-iNCH Disk after Both Surfaces had been Fine-Ground and Polished 



PLATE XCVII 





Fig. 1 



Fig. 2 





Fig. 3 Fig- 1 

Various Mirror Combinations in 60-inch Reflecting Telescope 



PLATE XCVIII 




Mounting of 60-inch Reflecting Telescope 
Under construction in Pasadena instrument shop of the Solar Observatory 




(^ 3 



S ^ 




;?; 


^ 


o 




a 


•tS 






'^ 



>> 




OJ 


H 


^ 


'A 


aj 






O 


ctf 


§ 


a, 


55 


CC 


O 


br 


W 


fl 


3h 


^ 


o 


^ 


w 


ri 


^1 


^ 




a 
o 
o 










s 




^ 




o 


fe 


^ 














:/2 









-q 




H 



I 




ft 



MAY 21 19Q8 




"oo' 




'^/"c 






'^^' ^; 



"»*« ,,^*j'' ^ 

^-">..-'.. 




'^v. o^^ 



-n/. v^ 



o' 













o 0' 



^:.>*^ 






o 0^ 






..s^\'\ 






^^^ ^' 






>-^ - ■ "^o r^^ 















m 



,-v 



oo^ 






''#«.^.* /'^ 



, \ 






o^.vs-..,-*^,; = ^" 



?.-^_ 



1^^'^ 
>^^'^^ 



ly* » 









^ /\ 



^ % 



N-^ ^^ 



<' -f* 






j^f^ 



^ c^ 







%. 







%^^ 



'T:^^; 



oo^^ 












■.^^ % 



. X - ..G^ 



■ '^"'^ -'.^^ -^ ^ 



.A 



i-^ ^. 



c^. 



* -^ 



x\^' ^/>. 






,0- 



A ' ^: 



^^^^Z- 



vV '.A, 






